MOTS is based on it being a unique forum group, just like something like an MIT, this is why MOTS are able to change this custom title while regular users can not as a little something for becoming one. They may change it as much as they please during the time they are MOTS as it would be like changing anything else on their profile like a signature or avatar.
In order for former MOTS would be able to gain permanent access to this function also means we would need to make another group called something like "former MOTS" that is hidden, and dump all previous and upcoming MOTS into just so they can have permanent access to this one forum power. It's not like this would be considered overwhelming work by any means, but for the sake of gaining permanent access to custom titles when it's a seasonal matter I'm not fully leaning into it.
Personally in my eyes MOTS is MOTS, and this function reflects it by giving these users access to it during the time they are one, and by making it permanent you also must keep in mind it will be less special if it's made permanent even if it's just the old and current MOTS. Staff can always remove a custom title MOTS has applied at a point if they do not like it or find it less suiting so it's not a custom title has to be permanent and not more than a few private messages away.