The only thing that I personally see maaaaaybeee happening one day is if the Team Comtress fixes get included into the main TF2 build. That's something that seems realistic. Everyone hopping to S2 TF2 and play that instead, if it happens, feels like a fantasy.
Not sure how S2 and its physics function, I've not delved into it, but if for example the port to S2 "fixes" surfing, that'd suck, so not only do you have to somehow port the main game, you also have to recreate the weird source quirks. I'd hope whoever is making S2 port of TF2 doesn't pull a crowbar collective and fixes quirks that needn't be fixed and then stubbornly refuses to roll those back.
I'm not holding my breath for S2 TF2, I'm only hoping for TC2 getting merged with TF2 so we get those fixes and changes Valve never bothered to do.