*spoilers*
It is only a rumour that the show will end after season 7, this is all because that Nick brendon (Xander) hinted that the season 'might' be ending after the 7th season. Plus, it is also widely speculated that UPN are in talks with Joss Whedon to produce another spin off, so even if Buffy itself does end, there will be plenty of Buffy spin off to watch.
One spin off which is in production is 'Ripper' in which Giles is the central character, and it is set in England. This spin off has been confirmed and will probably hit TV screens next year.
Another spin off which is rumoured is a Dawn spin off. It has been rumoured that she will become a Slayer, seeing as she has Buffy's blood.
One last spin off which has been rumored to be being mulled over by Marti Noxon (2nd in command from Joss
) is a unique one off series in which each episode explores the death of a Slayer in the past.
In my opinion, I don't think that UPN would just 'let Buffy go' if it is still bringing in the ratings, and even if Buffy did end, there will be loadsa spin off to watch, which will probably feature the main characters from Buffy at one time or another.