I don't think it is just children who want more, I thought it was an absolute prerequisite for a consumer society and all of us need to have consumption fever. Don't collect plaggy figures if you disagree buy ISAs,bonds,gold bullion etc and squirrel away your vast accumulated sums
Consumo ergo sum. Like love it makes the world go round.(and unhappy )
If there is a mismatch with the style or size of new sets and old sets that could be combined or used in the same scenario then it makes sense to revise some of the earlier sets. On the other hand some ranges have an integrity and to revise odd sets may provoke urgent pleas for the rest to be done.
Personally I wish some manufacturer would tackle the second half of the 20th century again which seems a no go area now though Esci, Airfix, Revell, Caesar etc all did sets of that period but have now ceased or succumbed to lobbying.
Nothing wrong at all with manufacturers coming up with their own ideas though it seems clear that most collectors want their niche interests satiated first or a tantrum will be thrown .