First off, To get this out of the way, Yes. It is super easy to clear some space in your inbox. However it's not always easy to tell someone that their inbox is full.
The point of the star poster medal is to notify people that that person knows what they are talking about, And that they can be helpful because they understand the forums and the game.
Mods have increased inbox space for the same reason that I am suggesting this. It's so that you can always contact them if you need to.
In the same way, You should always be able to contact a star poster if you need help because that is kmda what they are here for.
Obviously they should not have the same inbox space as mods.
I also understand why PMs are limited in the first place. However considering the fact that if everyone on here had a full inbox, We'd have at least 876000 PMs stored, I don't think that the 2800 extra (If we got 400 PMs) would make much of a difference.
I also don't think that extra space would be wasted seeing as star posters are considered valuable members of the community.
That being said, I would be lying through my teeth if I said that my only reason for wanting this is so that star posters can always be able to be contacted. I do genuinely think this would be a great reason to do this though.