By turning the trigger to a may ability you avoid possible feel-bad scenario's (like with a card that says "deal 4 damage to target creature", and you being the only player with creatures).
IMO, this is gameplay that you as a designer actually
want to happen. Imagine if I made a warp removal spell. If I warp that spell, my opponent has to choose to play their relevant creature they know will die, or hold it and wait out the kill spell. If my opponent chooses to wait, and warp is a may ability, I can opt to draw the card instead and continue to hold up the spell. Making the cast optional removes a lot of counter play, which is why I assume Suspend uses a similar template.
I don't quite get why it was a replacement effect anyway. I didn't read the discussion on mtgs, so maybe there's a valid reason for this, but I would just make it trigger on becoming the top card of a player's library. I think this avoids difficult rules questions like, what happens to the card that is on top of your library as the warped card would become the top card. Is it frozen on top of your library while the ability is on the stack? Is it in your hand, but not really? What if there's two warped cards on top of each other, which one resolves first? I imagine there are some horrible humility-level headache inducing questions to be answered, why not keep it simple?
If Warp triggered, it would mean multiple-card draw effects would not let you cast the spells. If I have a Warp second from the top, and I cast Divination, the replacement effect ensures that I will draw a card, cast the warp, then draw my second card. If Warp was a trigger, I would draw a card, trigger warp, then draw the warp with my warp trigger fizzling. We chose to go with the version that matched (what we feel would be) most player's assumptions and adding more interaction.