Here's the secret.
At low levels your save DC is going to suck, no matter what you do. Meanwhile, hitpoint-related spells such as Sleep and Fireball rock.
DC is going to suck, yet you will still rely on it fairly consistently. Sleep is great at level 1, good at level 2, and starting to become obsolete by level 3. Most of the other spells you cast are either going to require a saving throw, or a to hit roll. In either case, a higher Int will have consistent payoff.
But at high levels, your save DC can and will shut down just about anyone and anything, as long as you make smart decisions (big stupid brutes should be targeted by Intelligence and Charisma saves, not Strength and Constitution saves etc). Very very few monsters have great save bonuses for all six abilities.(snip) And at high levels, it again doesn't matter that much if you're sporting a 20 or just an 18... because once your save DC reaches 17 or thereabouts, its much more about avoiding the (few) strong saves of your opponent than worrying about that 17 not being an 18...
Depends on what you mean by high level. My VERY high level experience is pretty limited - so I won't claim to have expertise. At moderately-high levels, like 12 for example, with a 20 Int, you have a DC 17. Even creatures with a save of +0 will save 20% of the time. Also note that there are decidedly few spells that target Int saves (your example). Feeblemind may not be such a gamechanger against creatures that already sport a low Int, and Phantasmal Force is OK, but hardly impressive at high levels. Keep in mind that also you are often facing magic resistance at high levels, which means advantage on those saves.
So focussing on your spellcasting ability (Int in this case) is actually much less important in this edition than any previous.
This is the point I particularly want to disagree with.
Take a look at my 3.5 guide and you will find that I was not a proponent of more Int at all costs - other guides recommended the Grey Elf for the Int bonus, my guide did not - precisely because so many no-save spells were hugely effective. You could make a mid-level wizard who would never provide a saving throw for any of their spells, be massively effective, and that Int score was mainly for the skills. Many of those spells now either don't exist, or provide saving throws. Furthermore - gone is the near auto-hit of the touch-spell, and now your to-hit bonus makes a huge difference, whether just using a cantrip, or more powerful blasting spells like scorching ray or disintegrate, or effect spells like ray of enfeeblement. This makes Int important not just for DC's, but also for chance to hit.
At low levels, it doesn't matter much, since you have better ways of using your very few spell slots than to lock down a monster for a single round maybe.
I agree that is not a good use of a spell slot. I disagree that the alternatives don't rely on DC or To Hit.
In summary, those wizard guides are flatly wrong. They're written with a lot of preconcieved baggage from editions previous. Don't get me wrong, I completely understand it, and I too thought getting up your spellcasting ability score was the most important thing for a long time.
Again, I am not just in disagreement, I actually hold the complete opposite opinion. Int score is more important in 5e than previous editions in my view. As I reviewed the spells, I increasingly realized that Int had become more important than to wizards of previous editions.