I would much rather they get things working properly in a game without multi-classing, rather than scrambling to bolt it on (with all it's added complexity and balance issues...minefield) and yield a lower quality experience in the end. If they implement all 5E classes and races (and maybe borrow from some other non core classes, published by WotC) then that is already a huge amount of supported play styles. A year is not that long really....Even though 5E is much simplified compared to PF or 3.5E, it's still a big system to implement faithfully.