OK so if Tom Riddle had survived, Voldemort would be way more powerful... why?
What did Tom discover/ know that Voldemort does not?
Maybe Tom discovered / knew that Lily protected Harry with that Old Magic: Love...??
I reckon that Voldemort thinks that Harry had some kind of protection spell that Lily gave Harry by her sacrifice (he says this in the graveyard in GoF). Voldemort doesn't say anything about love. Therefore, he hasn't learnt, as Tom did, that this love/ Old Magic, is stronger than he can imagine or deal with.
We discover in CoS, that Tom is a totally independent being from Voldemort. He finds out Voldemort's/ his, history from Ginny; he doesn't 'remember'/ 'know' any of it!. Therefore, Voldemort does not know anything that Tom discovered. Thus, Voldemort hasn't learnt, as Tom did, that this love/ Old Magic, is stronger than he can imagine or deal with.

Jo's told us that the primary question we should be asking ourselves is "Why did the curse rebound?"

Voldemort still doesn't understand what he's up against ; love. He still underestimates the power of harry's arsenal -; his Mum's CONTINUED protective love - and the scar is not only a symbol of Voldemort's ignorance, it is a warning to him! Harry is still protected. Therefore, what if Tom had won ? BAD BAD Things - a Voldemort that would finally appreciate the power of Harry!! Fortunately, Tom no longer exists (that we know of!). Therefore, Voldemort STILL has no idea what he's up against.

Sorry this point is SO garbled but it's been a heck-ova-day! BUT:
What do you think?
What do you think of the idea that only Tom knew the truth of why the curse backfired but Voldemort does not?