The only problem having watch hours is the platform cant make any revenue if this were the case. So more streamers would be able to be part of T500 programs but the platform would not be making money to be maintained and continue operating. The platform would have more money out than actually coming in. Streaming platforms are not cheap to maintain or operate and going back to watch time payouts would indeed put the platform in a bad situation down the road. I would imagine the platform would need to expand much more to safely payout for watchtime hours and additional streamers to partner.
T500 programs with different requirements builds a challenge for different goals to achieve as a streamer. Providing variety and keeping it interesting. While some of it is hard or not fun, growing as a streamer, hitting different goals and advancing to new tiers once again keeps variety of different ways to earn as a streamer.