No problem. ModelSim is pretty much industry-standard, so learning how to use it will be reusable outside of the Lattice world.
On a side note, Lattice Diamond is looking like it's going to end up as Xilinx ISE. They haven't updated it in over 2 years (IIRC) and all new chips are supported in Lattice Radiant, which seems to be the focus of their efforts now.
So it's likely that for those sticking to the ECP5 and MachXO/2/3 series, the situation is going to be the same as with Xilinx for the 6-series. You'll be stuck with software that is probably not going to be maintained anymore, just like Xilinx ISE.
That's a bit of speculation though and Lattice may prove me wrong, but I'd say it looks rather likely.