I'm going to leave out the portions of the post that are just mud-slinging with no point to it, or are anecdotal at best and not actually in debate at worst, to just address some facts.
Once you get invisibility...
IF, not once. Chance to learn spell, and the DM being the one picking your spells and that not actually being directly tied to level in most cases, rather than the more modern way of choosing 2 spells every time you gain a level.
Caster power was factually immense in 1E and 2E. That is why they leveled slower.
Except for that sometimes they didn't actually level slower because of how XP was earned (by magic items in 1st edition, or via the optional individual character rewards in 2nd edition).
You bring up haste like that was what casters used.
No, I brought up
haste as an example of a spell that used to have a prohibitive enough drawback that it
wasn't a go-to spell even though its effects were extremely potent, that had the drawback removed (as many other spells with built-in drawbacks did) in the change to 3rd edition.
Then there were the save or die or save or suck spells.
Many of which you could only expect about a 15% chance of success with at high levels given the saving throw system and magic resistance rules.
How well you play a caster most assuredly does decide how often you end up in battle.
It influences it, yes, but not even the most carefully and intelligently played caster can be 100% flawless and lucky enough not to have to face creatures with their own abilities which counter those of the caster - for example, your use of
invisibility to avoid combat failing full-stop because the things which would combat you can see invisible things, or otherwise detect you just fine despite not being able to see you.
It's been a long, long time since I played 1E and 2E.
Incidentally, it has been less than a year since I played AD&D.
High level 1E and 2E casters were forces to be reckoned with, especially if you knew what you were doing.
That happens to apply to 5th edition casters as well, and also not actually contradict anything I've said on the matter.
1E/2E casters could run back and cast.
If you mean they could run back on this turn and cast on their next, you are correct. If you mean they could run back and cast both on this turn, that is not in agreement with what the rules in the book state on the matter, even if it was how you and your group played it at the time.
Just because you have to take a possible hit was meaningless
Since our entire conversation was started by you saying there were no opportunity attacks, and me saying that didn't mean there wasn't effectively the same thing, but also actually worse because you ended up in the situation where your declared actions for the round are locked in place, and the initiative roll screws up your plan: you wanted to cast a spell, but an enemy walked up and attacked you and you lost the spell slot, then the next round you declared your intent to flee and got attacked again whether you won initiative or not, and had to have someone or something stopping your enemy from closing the distance again, then in the third round of this example you again declare you are casting a spell and are likely at the mercy of initiative roll or DM "the monster attacks someone else because reasons" mercy.
You are confused as to how concentration applies in this situation.
I'm not actually confused about how concentration, or it's equivalent, applies in any situation for any version of D&D.
More powerful than a caster in 5E could hope to be.
Perhaps in an objective sense of comparing things like how a 2nd edition wizard could cast more than 1 9th level spell in a single day, but I think in a subjective sense comparing the power of a caster to their potential opposition from the monster/monstrous manual, that the 5th edition caster would come out as equally or more potent because of the differences in the parts of the game found outside the class and spells chapters.
You sure do not recall the 1E and 2E magic system other than this "If you run back, you get hit." So what?
Again, this entire discussion started because you said "there weren't opportunity attacks so you could run back and cast", so the only bit of knowledge about 1E and 2E magic system that is
relevant is that if you run back, you get hit and/or followed before you actually have a chance to cast any spells unless you had someone block the enemy from moving freely.
That is it. That's all there is to the discussion besides you slinging mud and ranting about other half-remembered irrelevancies and me, rather unwisely I'll admit, providing answer.
You said there weren't opportunity attacks, and you are right about that - I've never said you weren't.
You said that lack of opportunity attacks meant casters could "run back and cast", and I provided evidence of that statement being inaccurate.
And now, I am going to excuse myself from this mud-pit and go wash up.