Well, you do have a point lol, I am trying to be optimistic as a fan, although I think I might have a reason for it.
Osi and Strahan played most the downs, while Tuck was used on the inside (mostly on passing downs). For the most part, Osi and Strahan were not part of a rotation. The rotation mainly took place for the DTs (Barry CoField, Fred Robbins, and Jay Alford). Just pointing that out.
Now we have no idea if the other guys are going to be able to handle their new rules, so I'll reserve judgement on that one. However, that won't stop me from predicting.
Justin Tuck is going to play every down. That is a given. He has a lot of talent, but now with Osi out he'll probably be seeing a lot of double team. This will put the pressure on many of the other guys to perform. Kiwanuka will probably play mostly on passing downs if he is moved back to DE (because as you stated he's undersized). Renaldo Wynn and Dave Tollefson will also be rotated in and out, and what they contribute will be key.
Now, believe me, you would be hard pressed to find a Giant fans that appreciated more what Strahan and Osi accomplished the last couple of years. However, they've been together for a few years now, so what changed last year that allowed the pass rush as a whole to take it to the next level? The answer, in my opinion, is Steve Spagnuolo. I'm not saying he's the next NFL genius, but I believe his blitz schemes had a lot to do with all the sacks last years. They still have talent and I think Spagnuolo will be able to take advantage of that.
I want to say again though that losing Osi is a huge blow. He is far and away their best pass rusher, and his natural ability to get to the QB (regardless of the schemes) is a big loss. With half the secondary out him and Strahan took over the Dallas game last year, basically winning it themselves. They'll definitely miss that. All I'm saying is that I think they have some nice options in reserve.