As bitcoin becomes more mainstream, is it reasonable to expect that more and more bitcoins will be lost?
Yes. Through accidentally misplaced wallet keys, death, natural disasters etc.
And that even one day, all of bitcoin may be lost?
Possible. I don't foresee that ever happening though. For this to happen, everyone has to collectively either burn or lose their Bitcoins. It could happen after 2140 but you can be sure the more scarce Bitcoin becomes, the more careful everyone is.
Losing Bitcoin would just decrease the total supply and theoretically making every coin more valuable. There really isn't a negative effect to Bitcoin in general.