Easy. What you wrote only works out if spells scale by caster level rather than slot level. This is an example of a 5e spell still pegged to scale by caster level using concentration and short duration to keep it from being too good when it does. The system used in 5e however is to scale by slot level. The upcast carries additional cost that the spell does not compensate for.
I have no idea what you are talking about. Nobody has mentioned spells scaling by casting level until you said it right here out of left field.
Spells not scaling by caster level is not relevant to what I am saying, nor is it relevant to what humble minion is saying.
I understand that concentration is a resource. When I compare Bless against other concentration spells, there is no 1st level concentration spell that competes with it. Even a 3rd level concentration spells that many people consider good - haste - has a similar damage impact that bless has!
Bless is a 1st level slot. If you don't have higher level slots to spare, Bless is a great use of your concentration slot. It both gives a good offensive boost and a good defensive one.
I have explicitly described what the offensive boost is. I haven't relied on "this doesn't feel like it is powerful" or "this feels like this powerful". I noted that so long as you miss on a 1 2 3 or 4, then 1 in 8 attack attempts are turned from a miss into a hit.
If you cast bless on yourself and 2 friends and keep it up for 3 rounds, you'll get 8 player-turns worth of actions.
So you'll do damage
as if a player made an attack and had a 100% hit rate 0% crit rate on average, over 3 rounds. It costs you your action.
Thus, so long as you aren't a significantly better damage dealer than your allies, or your concentration doesn't drop immediately, then Bless will give you a damage boost over even a 3 round combat. If your allies do more damage per action, then Bless is even better.
It is true that Bless scales
better the lower your accuracy is. But even if you and your allies hit on a 5+, Bless over 3 rounds is better than attacking by almost a 33% margin. If you hit on a 12+ it becomes 100% better than using your action to make attacks (assuming you are about average in terms of attack oomph), as you give up (1 round of 45% chance to hit 5% chance to crit) for (a full PCs worth of attacks that all hit), which is more than twice as much damage.
And all of this is before we take into account the +1d4 to Saves.
I'm not assuming low accuracy. I'm not assuming spells scale with level. I'm not assuming anything I'm not mentioning. Please don't start putting words into my mouth and saying "this is only if spells scale with level" or "this would only be good if your accuracy wasn't high". The spell is good if you have 80% accuracy, and becomes
even better if your accuracy drops lower. But at 80% accuracy (hit on an 5+) it is worth casting! At higher accuracy, it may still be decent -- but honestly, situations where you hit on lower than a 5 aren't that common.
Bless is a good buff. It isn't the best most powerful concentration spell in the game. But it is also a 1st level slot; so if you don't intend on burning a 3rd 4th or 5th level slot on a concentration spell right now, burning a 1st level slot on an efficient buff is a decent plan.
Bless is a good buff. If your allies exploit the accuracy boost by using abilities like GWM or SS, it becomes even better. This is because Bless boosts effectiveness (as a percentage of unblessed) larger the less accurate you are. GWM/SS can switch from being marginal to great, or if great can already become better, if you have Bless up.
This is something I have checked in actual play. I have experienced Bless on a PC at both low and high levels. What more, I had a high level PC that got ahold of a pile of Potions of Heroism, and boy was that broken good (concentration free bless!)
In 5 minute adventuring days, by high levels Bless falls off in utility, simply because every fight you can afford to drop multiple high level slots on concentration spells. My bard would sometimes cast polymorph for less than a single round just for the purpose of tanking, then drop it before the cleric's turn so the beat up target could get healed. But bless was still a go-to buff even in the low teens, when 3rd level slots still had a price.