There is no pressure because people know performances in FC cricket matter less than U-19 and now T20. County Cricket is the gold standard of professional cricket for foreign players, and it's a great finishing school on honing your skills and becoming an international class players, yet the crowds are also sparse there. But virtually everyone who does a stint there is a better player for it.
On the other hand, I don't really care if my fast bowler can bowl seven slower balls but takes no wickets. You're retarding their skills by making them play in a format which rewards different skills for success. It's why many spinners shouldn't start off bowling in ODIs, it inevitably makes them flatter.
Frankly, this whole business of 'pressure' is overrated. You need skills first, and you need to hone them. If you are a skilled person who knows your game inside and out through a tough FC grind, I'd back you to perform better than a newbie with a few T20 wickets, I don't care how many people were watching when you got them.
And no, I don't think anything in T20 or OD tests your temperament anywhere near as much as proper four-five day cricket game. As a bowler, or as a batsman. Again, if you have a problem with FC pitches, fix is pretty straightforward. If you want more international stars, the fix is more costly but certainly doable. By having a system that in effect bypasses the FC system results in cricketers with ******** temperaments and blunted skills - players who don't know their game and can't adapt when the pressure is truly on. Just because you can smack a few around the ground, or bowl a couple overs doesn't mean you know anything about proper temperament of a long innings, or how to go about batting from behind. Or how to come back spell after spell even if you got carted in your first. Or adapting your game constantly session by session to the ebb and flow of the game when your body is tired.