Well, this may have been posted before but the original design railsounds audio boards with removable sound chips are definitely a thing of the past. The new Legacy audio board incorporate programmable eprom chips and are not removable. Guess all the remaining RS boards that are for sale will be obsolete shortly.
So, if you have a favorite engine with a RS 3-4 board in it you might consider buying a replacement, if available, just in case. I would venture that the RS5.0 boards would be available sometime in the near future. Lionel does not need to hold on to parts after the three year production period according to their warranty.
I'm not sure what Lionel's policy will be going forward to replace defective RS boards when the parts are no longer available.
Might be a lot of shelf queens and door stops in the future. Maybe just a lot of conventional engines for cheap with no sound just a reverse unit.