He is not losing that many votes. These Cuban Americans are captive to GOP. High time Democrats stop pursuing the vote they are never going to get.
There are two parts to making big moves: you may lose swing voters (which in this case is pretty unlikely) but just as important you may motivate fringe voters to go vote in greater numbers (which in this case is pretty likely.) If you don't gain voters, but manage to get more people to vote against you, that's a big deal in political calculus. Of course, the opposite also happens -- by doing something big you may motivate fringe voters on your side to come out and vote for you (which is arguably how Obama got elected in the first place, along with running against terrible opponents) but the number of people who feel very positive about restoring relations with Cuba is extremely small compared to the number of people who will be infuriated by this, I suspect. It's a single issue voter thing.