Just a little business strategy thinking... it's going to be pretty tough to get any miners on your pool if it starts out with Bitcoin mining. If I were to start a mining pool, I'd aim to build the #1 mining pool for some mid-level altcoin, which probably could be easily accomplished with just some good web design and discussion in that altcoin's community for a few weeks. Then, you'll have a core set of fans and followers and can branch out into larger coins.
Yeah you're right, from a strategic point of view it makes sense.
But there is a real market with small Bitcoin pools. Especially now that mmpool.org doesn't exist anymore, we still have the Kano pool, and the solo pool of CK.
Apart from that there is Laurentiapool, but their membership system is not intuitive I think. At my knowledges there is no other very small pool existing?
A lot of people like to send a lot of shares to small pools via rental (MRR or Nicehash), and then let an S19 or something else run on it until the block solved. Philipma has already written about this on the forum and he is right. There are good potential profits to be made with small pools, especially if after doing what I described above, the block is found quickly with a lot of luck, it can generate good payouts quickly.
Personally I will always welcome a new small pool with open arms, but there will always be the question of the legitimacy of the owner. On that point you are totally right, but OP could very well pay dozens of altcoins blocks to its users, and then exit scam with the first BTC block when it switches to BTC. It wouldn't give any guarantee even with some success with altcoins blocks.
Small pools are always a risk for miners. (mmpool example is a perfect example unfortunately, of why small pools are a risk for miners...)