That's certainly AN ending but I don't see that it needs to be THE ending. Like... Okay, bear with me here, I'm going to be a total fucking dork and compare Summertime Saga to minecraft for a second.
Minecraft has been in active development for like ten years, right? For the longest time it was just a sandbox. Build, dig, mine, collect. Then partway into development it got a boss and an ending screen. That wasn't the end of the game and it was definitely not the end of development though. You can keep playing well past that point, and a fuckload of content has been added to the game since then.
Same deal with SS, right? Yeah we could have the prom thing constitute AN ending to the game complete with a cutscene and credits and all that but there's no reason for the game to be over there. Continue on with the sandbox. Prom is over, dump that bitch and move on, keep adding new girls, new systems, new locations, etc. Just like minecraft, development on SS could theoretically continue on forever if they wanted it to. That's not, as I see it, patreon milking. That's the nature of a sandbox game.