It's probably "possible" to change all these entity types to make use of dictionaries with sub-entities instead. But even if ADesk goes through all that trouble just to move a "linked list" structure from one storage position into another (for arguably little benefit, if not worse) ... they'd still need to keep SEQEND alive just for backwards compatibility.
So killing it off is rather a misguided idea. It works well for those entities where it's used, so leave it be. For those where it might not work so well (i.e. sub-entities not simply being a list of "owned" entities, but something like a tree structure of subents with their own sub-subents recursing ever deeper) you could always use dictionaries. And it's not as if SEQEND is actually "bad", it's at worst just as much extra bloat as a dictionary object attached to a container entity, usually a dictionary is lots of times more space hungry though.