I have allot of problems with Reach, but Sprint did not invalidate its core "Halo-ness" it simply didn't. Yeah, it changed the "gameplay" but as I already covered and I'm sure you agree, you can't judge something simply because it changes Halo's gameplay.
I was playing Big team Slayer on Sandtrap in H3 just a few hours ago...boy, it felt good. And looking back, if I could sprint somewhere and everyone else could, would that really alter my feelings about the game? No. At worst, we can say Sprint isn't really a good or bad feature. You have this notion for what "Halo" gameplay is and I challenge you to define it. For me its a few things: A good degree of symmetry on both teams, good gameplay and aesthetically pleasing map design, straightforward vehicular combat, etc etc. I can go on. If Reach went wrong with Sprint at all, it was maybe not making it universal. If 4 has Sprint for everyone and now no one can choose Armor Lock, I will be happy. Point is, Sprint does not inherently go against what is Halo gameplay. You seem to disagree, so identify what element it is thats so important and then explain how Sprint contradicts that.
Also, in sales, Reach beat Halo 3. It was not a flop. I think allot of Halo fans, myself included, agree that Reach was definitely not all it could be, but don't throw the word flop around because A) Reach wasn't a flop and how well it did financially doesn't have that much to do with how good a game it is.