Sounds a bit like my brother. I hate going shopping with him, don't buy that, that's a waste, you'll never use it blah blah blah. Usually we leave with nothing then I order it online anyway.
Sometimes, when you decide you want something it's best to just get it, forget anyone elses opinion and just please yourself.
I actually told my brother off last week, he's getting a new monitor and was bragging about the response time... well I just told him straight that I could not give a monkey's chuff, and that he should just enjoy the new monitor, instead of turning it into a bitch race every time.
So yeah, go for it - you've decided that you'd like a new processor, one that will last a while, and your brother probably can't afford one - so he can't see the relevance of spending more money on future proofing. Ignore him unless he has major concerns - but mostly it's just ill-educated nay-saying. The best thing to do is buy it, then don't mention it at all to your brother, he'll know you have it, but don't brag about it - be humble and it'll annoy the hell out of him even more. If he is like my brother, then he'll be hoping you brag about it so he can tell you how 'pointless' it is. Enjoy your purchases, and know that your not the only one with a buzzkilling-naysaying-christmasruinning brother.