Dalamud, Cait Sith and Bahamut meant that all attempts at a complete collection are inherently futile. And the Yo-kai Watch minions the Primogs and Excalibur minions would be added to this list for players that joined later. So I'd still be collecting minions regardless.

The thing is, this was inevitable that they would add more time gated minions. The more a game gets updated, the more things are gonna be added and the more likely that it will be impossible to get all the things. There isn't much that could change that. A good example would be the Trading card games such as Pokemon, Yu-Gi-Oh and Magic, where the individual cards would number in the thousands at best and the millions at worst (plus some very rare cards as well)