1. Since his best Weapon Proficiencies are STR based (A.K.A non Finesse) and he needs at least 15 STR to wear a Full Plate without speed penalty I classify him as a STR based class.
2. Speed Penalty, I believe your archer don't have 15 STR
3. Since most of the Warlock's Features (A.K.A spells) are CHA based not using his prime attribute goes against his nature. Yes he can use a Blade, you're just dumping your prime attribute.
4. Tell me the advantages then
5. Positioning issues only; most of the time it was AWAY safer just to Eldritch Blast instead of getting in melee range.
In practice, when the game is running, usually somebody or something must hold the enemies away from the rest of the party. Assuming the party does not have access to something, such as some area control spells, collective enhanced mobility, etc, that would effectively result in all melee types being obsolete, this somebody who will hold enemies away is better served by being a capable melee PC. If someone is choosing to play as a bladelock, they are probably making the conscious decision to be that someone who will hold the line in melee to allow for the squishier in the party to be safe. If being in melee is not an issue, why even bother discussing bladelocks, or any other melee build from any other class for that sake?
So, for the rest of my argumentation, I will assume being in melee is somehow relevant when playing D&D, just to be able to keep the discussion meaningful in any way. I guess discussing the merits of being in melee is a topic for another thread.
Without feats, the eldritch blaster is not a real top-tier threat in melee. They attack with disadvantage, their primary atribute (CHA) does not contribute to their defense in general, they have to invest in invocations to keep up damage with any weapon-user from the very beginning, their opportunity attack suck. A bladelock, on the other hand, might use STR or DEX as their primary atribute. This way they can choose to be more offensive, if going STR and big weapons, or defensive, by going DEX and using finesse weapons. Now they attack without disadvantage in melee, and have a ok-ish opportunity attack to help them fulfil their role. The STR-based will have a similar AC as the Cha based eldritch blaster, and the DEX-based will eventually end up with better AC. CHA not being primary definitely changes the way this warlock will handle their own spell list, and they will generally be less effective offensively spell-wise, but one cannot have everything, right? There is always a compromise. Anyhow, there are some funny stuff warlocks can do, for example with armor of agathys and some clever use of blade ward, and when it is over, hopefully enemies are already starting to die, and you can begin benefiting from dark one's blessing.
With feats, eldritch blasters can be effective frontliners, but it has.... guess what? - a price to pay, as everything in the game. Crossbow expert just to get rid of disadvantage in melee, plus warcaster for that extra oomph opportunity attack, which I am not even sure could be used with eldritch blast, as this spell can potentially target more than one creature. Now, STR and DEX bladelocks have other nice options, unavailable for eldritch blasters, and they can also benefit from warcaster, by making good use of booming blade. Some people point out that tomelock is a better melee warlock. This is only true if one is putting a premium in going CHA primary, and ultimately limits your melee options. No GWM or defensive duelist for you, for example, and limited array of magic weaponry.
The truth is, there is a lot of double standards, and there are a lot of compromises some people seem not to be willing to do. For instance, I see people complaining that the thirsting blade and life drinker invocations are "taxes". Well, you want a neat feature but don't want to pay anything for it? Do anybody really think valor bards, or even fighters, get extra attack "for free"? This is their sole feature for the level fighters get it, and is the one college feature bards get at that level. The fact that these features cost invocations for warlocks is a feature. Some bladelock multiclass builds might skip thirsting blade, for instance, because they will already get it somewhere else. I wish I could multiclass hunter ranger-champion fighter and not feel like I should go only 4 levels in one of the classes to avoid a completely dead level, the same way as I wish a valor bard would have a better synergy when multiclassing fighter. Moreover, wouldn't agonizing blast, by the same logic, be a "tax" for eldritch blasters? In the long run they will also probably get at least one of eldritch spear and repelling blast too, or even consider taking spell sniper to get rid of partial cover, which is a thing in many games.
For a simple STR build warlock using standard array and no optional rules, how about a maul-wielding mountain dwarf? STR 15 +2 = 17 DEX 13 CON 12 + 2 = 14 INT 8 WIS 10 CHA 14. At first level scale armor + DEX 13 = AC 15 is not awful, and at level 4 halfplate might be affordable, throw in +1 STR and DEX, AC goes to 17. Extra attack at level 5, you don't even look much different that your warrior counterparts, solid CON, less base HP but a lot of temp HPs all the time, extra damage from armor of agathys, might as well use booming blade as main attack, at least until level 5. CHA is not primary, but is far from dump. At level 8 you can max STR and at level 12 you may increase CHA and get lifedrinker. And you still get other invocations and one spell slot to play with, every short rest, assuming you are always casting up armor of agathys. Overall very solid build, in my opinion. A half elf or tiefling DEX based would be different by having somewhat less HP and base damage, but better CHA in general and AC on the long run, by taking armor of shadows invocation. With optional rules, a variant human can choose where to go, maybe picking medium armored or warcaster at level 1, being the stickiest melee right from the gate.
In the end, don't expect a bladelock to be as effective in melee as a STR champion fighter. They should never be. After all, when going bladelock and choosing STR or DEX as a primary stat, you are not giving up all your invocations, and when using armor of agathys to help absorb damage, you are not giving up all your arcane power. Actually, far from that. But don't go on thinking it is not a viable option, because for a lot of games it is. And it is awesome that it plays very differently from eldritch knights, valor bards and bladesingers, because the chassis is different, the compromises are different, the spell list is different, and more importantly, the flavor is different.
Edit: to address @
Leo Giorni Nocchi point about fighter being a STR-based class, well, in the optimization forums you will find a lot of people, I mean really, a lot, that disagree vehemently with you. Heck, there are advocates for DEX build paladins and even barbarians. Moreover, rangers get exactly the same weapon proficiencies as fighters and are most often considered DEX based, which goes strongly against your argument. Anyhow, being such a big fan on staying away from opposition as you seem to be, it is at least ironic that you support the opinion that the fighter is STR-based. It seems to me your approach to atributes, classes and character build is actually quite narrow.
