I will likely stay with FSX Steam Edition until MSFS is finished. I'm just not that fond of paying to be a beta tester and the frustration of a broken sim after an update sounds like the sort of frustration I can do without. Just my opinion.
Microsoft/Asobo have said they're planning a 10 year lifetime for MSFS in terms of updates and such, so it'll probably never be "finished", especially since the plan is to release updates for various countries/regions as Asobo gets better satellite and mesh data over the next few years.
According to the timeline they've laid out, MSFS will basically have all the basic features FSX/P3d did by some time next year (which will see the addition of native support for functioning aircraft carriers, gliders, and helicopters), so it's possible that the sim will be largely done in terms of adding new types of aircraft by next year, but there's probably a lot of "tweaking" that'll happen after that point as well.