In theory, games don't NEED to have a platinum trophy to be good or to increase sales, but they most certainly help. If I'm sure I'll like a game, of course I'll buy it regardless of a platinum trophy, but if I'm on the fence, or if there's a crazy sale, a platinum trophy will most likely put me over the edge to purchase it. I find it astounding that Devs still don't understand this. Sonic Mania and the Mega Man Legacy collections should most definitely have platinum trophies just based on game length and content.
Recent example, Night Trap has a platinum trophy even though the Devs thought it may not due to game length. But, now that it has one, it will certainly sell more copies and therefore reach more gamers which is the intent of creating games in the first place. People have various reasons for purchasing games, none of which are "wrong". If you're a Developer with the intent to profit off the game you make, you should want to touch on as many reasons people buy games as possible. Having a platinum trophy should be a no-brainer way to grab one segment of the gaming audience.