Probably the reason why we don't have the option to downgrade to an ME1 era weapon is that it wouldn't be very effective, like trying to kill a tank with a handgun. Only the tank armor is on an angry guy who's shooting at you.
This is, more or less, the explanation we're given.
We're told that it isn't so much how hard you hit a shield, as how often you hit it. So rapid-fire weapons became very powerful. And the built-in heatsinks couldn't keep up with the heat being put off from these guns anymore. So they basically built disposable heatsinks. Dump all your heat into a clip, get it good and toasty, and then eject it instead of waiting for it to cool off.
But that doesn't make much sense. You're throwing these clips away, when all they have to do is cool off. The clips you find scattered around the battlefield? They're generally other people's spent clips that've had a chance to cool off. So why not just hang onto your own clips and re-use them when they're cool?
Or, barring that, why not throw in a traditional heatsink in addition to the clips? Yes, I know, it can't keep up... So I can only fire one shot every once in a while... And it will not be effective for a real fight... But it'd give you some ability to continue fighting after you'd run out of clips.
Looking at this from a storyline perspective it just plain does not make sense.
From a gameplay perspective... I still don't like it, but it's at least acceptable.
I'd prefer to be able to keep firing even if my shots were going to be slower or ineffective. There's something psychologically satisfying about being able to keep shooting even if it isn't doing any good.
But the clips aren't a gamebreaker.
They just don't make sense.
I was astonished how bad the inventory system in the first game was. In the most RPGs it's nothing special, in ME1 I actually suffered every time I found some weapons from stray geth. I frequently used to run with inferior weapons just because I couldn't bring myself to rummage through this mess and check if Obliterator XXX is better than Masturbator XXIX. Also, the equipment seemed to be generated randomly (with some variations), so even searching the dirty, flea-ridden rags of a vorcha junk peddler near the end of the game would give me better weapons than a crate in the middle of a military research base. They could have at least thrown a "omni-gel all" button.
Yup.
I like the inventory management and loot hunting aspect of most RPGs. It's fun to find some uber weapon. It's fun to figure out who's going to get your one supergun. It's fun to go shopping and see how much you can afford to get and agonize over whether you should buy two good guns or one amazing one.
But ME1 was just plain awful.
Tons of loot everywhere. Most of it basically indistinguishable. Three different guns with just a single point or two of difference between them. Then you had the weapon mods... And the armor... All of it available in different styles and ranks... And none of it terribly different from anything else.
No quick and easy way to dig through your inventory. No good way to omnigel all the crap at once. And when you were over-full there was no way to skip over the really awesome thing you just picked up and delete something else instead.
Absolutely terrible.
But I don't think just getting rid of it like they did in ME2 was a good solution. I still would've liked to be able to go shopping for weapons and armor. I still would've liked to be able to equip different types of armor on my party members. I still would've liked inventory management and loot hunting to be an aspect of the game, instead of just vanishing completely.