Something tells me that the F-35 will not accept small arms fire as well as the A-10, let alone AAA. Down low, the bigger threat are bullets, not missiles.
The "Jack of all Trades" is more cost efficient since you don't have to train three different people on three different airframes. Not to mention parts for repairs and upgrading.
That being said, I do not think it is effective in the long run. While an argument can be made that we have the Apache, it is much slower and fly's lower than an A-10. This makes it far more susceptible to small arms and AAA. Not to mention is isn't as tough as the A-10, though you can't thumb your nose at it.
All in all, I think an upgrade to the A-10's, be it further improvement to the existing airframe or development of an entirely new airframe, is something that would be useful. Something with as much, if not better/more, durability and loiter time, as well as ordinance.