I would think one would simply replace things as possible if and when they die/break. If one is not completely committed to command control, one could operate PS2 and PS3 locos in conventional mode if the command equipment (TIU, WIU, remote) died.
If command is key, and the price of new or secondary market TIU/WIU equipment became out of reach or the items unavailable at some point, start converting favorite locos to TMCC, or, in the future, perhaps LC +2.0 if retrofits become available.
New locos, if desired, could be LC, LC+ or Legacy, which operate with inexpensive remotes or tablets, no TIU/WIU or Legacy base needed. Just spend $40 on a universal remote or use a smart device (phone or tablet) you already have to control new Lionel locos if you don't want to spring for the Legacy system.
Will almost certainly happen gradually, so cost doesn't become the main obstacle. Just a matter of preference as to the path to take if MTH (and DCS in new locos) actually are no more.
