If you're talking about guessing what the market price of Bitcoin would be in whatever your preferred local currency is if there hadn't been any hardforks, there's no easy way to know that. You can certainly speculate, but it's not possible to say with any real accuracy.
Yeah, I think that some people - including the hardforkers themselves - overestimated the willingness of people to completely abandon Bitcoin in favour of their forked coins, along with the different techs. Few would have just stopped using Bitcoin to keep on a sole Bitcoin Cash/Gold/etc path, wouldn't have made investment sense anyway.
Did the hardforks affect Bitcoin's price? Probably, but people might recall that Bitcoin shortly reached its ATH *after* the forks. Of course, the momentum was already there and the forkings probably added to Bitcoin's surge in mainstream awareness. But I don't think the forks had any more effect than alts did.