several scenarios:
1. it can be done in a year, if most web uses Ogg/Theora.
only IE and safari refuses to support theora, and arstechinica is saying some company is developing a silverlight plug in for IE to support theora. At that time, safari will be the only one that doesn't support theora. but since safari only has 4-5% marketshare, we can say "most people" will be able to use it.
time needed: ~1 year
2. it can be done with a new open codec, google is in process of buying On2, a codec company, if google were to release a free codec and use it in youtube, then everybody can support it. apple probably still won't be happy, but again, safari is a 4-5% marketshare browser, we can say "most people" again
time needed: ~ 1 year
3. it can be done with H.264, after MPEG-LA patent expire in 2028.
time needed: ~18 years
4. it can be done whatever way MS wants, once IE throw support behind any of the solutions, all other browsers will have to follow, native, or by plugins.
time needed: MS's choice
yeah, it will eventually be done, but during the process, who knows, flash might get even better, become a standard, and become OSS.