would be nice if dev. want back and remade all three games into one game...
would make things much easier for everyone.
There's several different advantages to having the seasons be separate.
The first is that it kept the download times fairly short in that you only needed to download the current season for the update. Obviously that particular benefit went away once the game was completed and there'd be no further updates.
The second advantage is that if you are coming to the game for the first time now, after completion, you can start playing season one while season 2 and 3 are still downloading if you want. That benefit really came into effect after the completion of each series, but remains.
The third is that it was always known that this is not the run of the mill VN. Some love it, some absolutely hate it. For those who decide to abandon it, well, they didn't have to waste time downloading the latter seasons. Download and play Series 1, and, once you are coming to the end, decide if you want Series 2, and repeat for Series 3 and 4.
It's no different really to how large files get 'chunked' for downloading easier pieces, or how torrent downloads prioritize the first scenes of a video so you can start watching it even before you have the whole thing.