I don't play the games, but periodically flooding with new girls must eventually dilute the whole thing. Same with Kantai.
A man can individually love 500 girls: but not 50,000.
Touhou does it a lot slower than Kancolle does (approximately five girls a year compared to Kancolle's approximately thirty), which gives the audience way more time to get used to each new character. This actually helps a lot with the whole dilution thing. Patience is a virtue, and ZUN understands that very well.
Also, fun fact: IIRC, the largest group of people that the human mind can comprehend is 250 people. So, to love 500 girls, you'd have to split them into two groups and think of them as being completely separate groups.
