They could have simply put stricter limits on grenades (1 MIRV, for instance, and only for the Demoman- the Heavy would get none; one gas grenade for the spy), and also changed some grenades, (for instance, let only Scouts have concs, and maybe make some sort of new grenade for the medic, perhaps one that creates a small infection in anybody the blast radius touches, and then when they touch someones else, the infection spreads, and becomes a little stronger each time, but can be cured by a friendly medic.) There were other options besides throwing veterans out the window. Adding the totally same grenade system would ruin the balance, yes, but a revamped one could do the game well.
It could very well add teamwork. Say you have a very heavily defended room, and you can't penetrate it alone, even with your grenade. But, if you get a few of your guys together, you can put together a coordinated assault:
(This entirely hypothetical. There are so many variables to take into consideration that I doubt it would ever go exactly like this. This is just a situation to demonstrate how a revamped grenade system could add to the game.)
When everyone charges in, the heavy could go in front and take some fire so everyone else does their thing. The engy could throw his single EMP in the general direction of a sentry, or group of sentries, to take them out for a second, buying some time. The demo could toss a MIRV into a group of heavies, to hurt them seriously. With the bulk of the defense gone, the attackers can then proceed to put up a good fight.
Of course, the defense could use grenades too. In all the chaos, a medic could toss a previously mentioned "Infection grenade" to seriously hurt the attackers- if he isn't hurt by one of the attackers. Of course, things might get a little spammy if a grenade explodes as soon after it hits the ground as in TFC, so it might be wise to make it take a little longer, so that the attackers can get out of dodge, thus forcing whoever is throwing said grenade, to actually think about where they throw their grenade, instead of a, "throw and forget" situation. They would have to take a longer fuse time into consideration so that they throw where they think the enemy may be- not where they are at that very moment. A grenade indicator like in Call of Duty (yes, I know I said this isn't COD, but a grenade indicator isn't exactly the most realistic thing. Perhaps a "shout warning" key, so that one could warn their team mates of a grenade.)
/end really really long hypothetical situation.
Yea, I'm no professional game designer, and I'm sure there are holes I never thought about or took into consideration, but that's my general idea. A revamped grenade system would make the veterans warm up to the game some more, while still keeping it like TF2 is right now.
I have gotten some serious playtime with TF2 now, and I admit, it is still an enjoyable game- but it just doesn't feel quite like a TF game to me. I still need a little more playtime with FF though. They have been changing stuff a little as well (they removed the medics infection spreading in a patch.) So it's not like it's a carbon copy of TFC, they've changed stuff too...