i think the simplest answer is, that since Nostradamus predicted Dracula will be at his strongest in 1999, the Belmonts decided that they should wait & bide their time so they too will produce an heir that will match & exceed draculas strength. maybe something like "this is when he'll be at his peak, and it is also when we can put an end to him forever." it was that rare astral alignment that gave them the clue to his ultimate defeat.
plus factor in richiter feeling shame for being manipulated/corrupted. perhaps they wanted to make sure that wont happen again for 1999. which is why eric said "the Belmonts cant touch the whip until 1999"
they know that despite being of holy lineage, their blood can still be corrupted. simon almost succumbed to the curse, soleiyu almost succumbed, and again with richiter. if it happened yet again in 1999 when the Great King of Terror was back, then bye bye human race
gotta feeling had that akumajo dracula ps360 game been released, we would have a concrete answer. it seemed to be a direct sequel to symphony