...If the IDs are in the order of tens of thousands, what do we have to worry about? We can practically 'waste' 1000 IDs on what is functionally one block, if we want (Kind of like Minecraft's technical blocks), and still have massive amounts of IDs left... Or have five hundred different styles of door (Plus door wedges for each) and 150 colors of hull with all corresponding shape blocks... Or even create a separate, medieval-themed gamemode and share the same block 'list,' all without even getting close to running out of room... So, I don't think that IDs should be a concern.
I think at anything above a limit of ~8000 blocks there's really not any number of future additions/systems within reason that could make adding more blocks sound like a bad idea. Right now we have 478 blocks (I think; that only counts Kupu's figure +1 for the shield capacity block). Adding every reasonable suggestion in the entire suggestion forum that doesn't contradict or functionally duplicate another one would give us another ~250-1600. (No idea how many unimplemented blocks the suggestion forum actually holds right now.) Even with that, there would still be enough IDs for... pretty much anything, even with several large decorative block mods installed.