The downfall of H-games can only be attributed to the same shit that shittyfied the rest of the internet. Monetization.
Those shitty flash games were good despite 0 artistic skill cause they were works of passion. Some horny asshole wanted to share what made him horny with the world and that's cool.
Now all everyone cares for is money. R34 went from a couple semi-shitposty softcore pics drawn on a napkin to proper university educated artists chasing twitter's algorithm for the next FOTW girl so they can get another thousand bucks on their patreon. Likewise, H-game devs simply DON'T CARE for their games. They're just a vehicle to escape wageslavery to them and as long as they stay unfinished, the money keeps rolling in month after month.