If this post seems like it straggles from having a main point or doesn't seem to flow right from one thought to another... it probably doesn't. I went back and forth from one thing to anther as I wrote this, but hopefully it's organized enough that it makes my point clear.
We OFTEN referred to "consecutive hits" in melee as "combos" because given the person's DI, there was nothing they could do to escape that next hit. They were still in hitstun. They hadn't hit the ground. They had no control over their character except for DI. This closely matches the traditional fighter definition. Traditional fighters do not have DI, so this was acceptable. Sure, depending on your foe's DI, you might not always be able to combo a certain attack into another attack. It was rare that anything was guaranteed, but with few exceptions, you could always combo that certain attack into some attack. In real matches, you'd probably find that the foe did escape hitstun for a brief moment in a lot of these big combos we see, but the window was small and they often didn't have enough time to throw out something in the attacker's face, unlike brawl. Or even if they did have enough time to throw something out, the comboer would have something which outprioritized or outranged them, and continued juggling, which many smashers still referred to as comboing. Melee was a game where air dodges could be punished, which gave people incentive not to airdodge except in the most extreme scenarios. Try playing melee and airdodge when you would airdodge in brawl. You'll avoid one hit and still get hit by another anyway (of if you're over the abyss, you'll die). Even when out of hitstun, players really didn't have many options while in the air. This is why people are wanting to call strings combos now: String has never been in the majority of smashers' vocabulary, because we've used combo for both combos and strings back in melee, back when people could do a whole lot less about strings than they can now. We've had some 7 or so years without ever having to need to really differentiate between the two.
There's no need to reinvent the definition of a combo. We already did that in melee. I dare you to find one combo video in melee that didn't have at least one string in it (and isn't retardedly short, either). If it's 5+ minutes long, then two strings. I guarantee you won't find one.
Anyway, in brawl, the hitstun has been mostly removed, making more and more "combos" strings. There are some combos, still (kirby: 0-40% combos against a ton of chars, pika: fair -> dsmash, fox: dair -> stuff, various CTs, etc, etc), but it's nigh impossible to link together combos like melee had. At some point they are going to be free from hitstun for long enough that they can knock you out of your "combo," and due to the new airdodge mechanics, getting consecutive hits off when the foe is out of hitstun has become a lot more impractical. Even still, there are certain scenarios that pop up due to DI, that do allow you to combo further. By the strictest definition of a combo, pika doing a u/dsmash -> thunder wouldn't be considered a combo, even though if they DI wrong, they are getting hit. Even if they DI right they can still get hit, although that isn't guaranteed. This is my very point, here: Defining a combo by its strictest definition when talking about smash is difficult to do, because due to DI, the way a foe is sent after various attacks changes. A certain attack may always set up for one of two attacks, which require different DI to get out of the third hit. If your foe uses the wrong DI, you get a third hit off. Combos in smash are dynamic. Very very few long "set in stone" combos exist in smash. Branches leading off of one opener, however, are common, or were, anyway, back in melee.
I'll agree with you that it shouldn't be called a combo if the foe has multiple chances to retaliate, and the retaliation would be effective (which, again, due to the new airdodge, probably would be). However, in a situation where if no matter what they did, even if out of hitstun, they were going to get hit, I think it still matches the definition of a combo. It's inescapable. Seeing as at higher levels it's rare that a foe won't retaliate if they have the chance to, this whole "what a combo means" debate is hardly important. You won't have to worry about whether something should be called a string or a combo at high levels, because either the foe is gong to be comboed, or they're going to knock you out of your string. Strings don't get long enough in brawl for someone to say "nice combo."