While he may have obtained popularity for his tiers, it now stands as a symbol of MvC2 and is popular for a different reason. Whether you know if sentinal is good, a lot of people know who sentinal is now because of all the spot light his high tier gave him. Casual players rarely have a good concept of tiers and will mistake an easy gimicky or fast character as OP and assume technical and slow ones as bad.
I doubt sentinal would return, but anyone bitching about old broken characters returning as god tiers has not done their homework on fighting games. A few small tweeks can be the difference between roll-tier and god-tier if you change the right things.
And yes this game is highly combo based. I feel even more now and with a lot more options. However the combos are not so timing strict as SSF4. These ones are generally easier but have more to them trading anal retentive dexterity for the ability to manage a lot of things in a small amount of time with hit confirms and what not.