Well, Rome wasn't built in a day, and if they begin accepting donations, y'know, it begins to become a bit more tangible. And are you sure SDK isn't open source? I could have sworn it was. Greedy ass Valve.
But yeah, EVERYTHING good in life requires work, why should a new engine be any different? I'm sure there are a lot of experts on how game engines work in this community, if we could get them to contribute, things might see a whole new trem, a BETTER trem (RAGDOLLS DAMN IT!)
And as for the increasing need for hardware upgrades to run games at sufficient quality: Get a job. Thats what I did, and thats what I've done up until this point. Sure, some people may live in high-cost areas and don't have the extra buck, but if know how to live frugally, you'll notice a few fiscal growth spurts.
Tremulous has taken seven years to reach it's current state. The developments you describe require somewhat greater work than what has already been done.
Valve, Id and other game houses pay many professional programmers and artists hefty wages to develop their games full-time. Even they take years to develop a game.
On a side-note, even if the Source SDK is open-source, you'll still need to license the source engine to distribute it.
As for improving the existing (Quake III based) engine.... Study the code. You'll see that it could well be easier to use an engine that was actually designed for the capabilities you want.
The community here indeed have much skill in engine coding. However, depending on their situation, they also have jobs to do, studies to complete, social lives, etc. among other activities higher up the priority ladder than video games.
To conclude, if you want this so much, there's nothing stopping you from making a start. There are plenty of open-source engines with the capabilities you want. Set up a website, get the foundations laid. Put some work in and get something working, a screenshot of a hollow-cube test map to demonstrate that you've assembled a basic engine should help here.
At this point you can start to accumulate a development team. Good luck.