Problems with decidability open (for a long time) proven decidable
I suspect there are many of them.  Pick almost any recent paper that shows decidability of some problem, and I suspect chances are good you'll  have an example.  So, I doubt your heuristic that if decidability remains open for a long time, chances are that the problem is undecidable.

For example: The matrix mortality problem for non-singular 2x2 matrices was recently shown to be decidable (Decidability of the Membership Problem for 2 × 2 integer matrices, Potapov, Semukhin).