Probably the thought that he would be forcing the most successful driver in F1 history out was the big temptation. The problem will only come if his performance in the Mercedes is as lack-lustre as Schumacher's.
Brawn is the Kingmaker and I guess Kewis is hoping he'll do for him what Brawn did for Schumey and Button. If nothing else it'll be interesting.
Of more interest, Lewis is a clever guy and one of the most knowledgeable drivers on technical matters, with much formal training given to him by McLaren. His wee brain will bring a wealth of technical data to Brawn and it may disadvantage McLaren as much as it benefits Mercedes.
Conversely, now it's known he is going McLaren will seek to isolate him from future developments, upgrades and work on next seasons car which may impact on his title chances this year.
I suspect the nearly double basic money Mercedes offered over Mclaren's best offer, plus the worldwide footprint of Mercedes, which will greatly increase his sponsorship and advertising income, has decided it. Also I can't think that Mercedes won't give him at least one season with a championship winning car in the next three years.
Historically I don't think there's a problem him leaving after six seasons. He's been given six years in McLaren in F1 (know he's had plenty of support before that).
Compare for various notably famous McLaren drivers apart from Bruce himself
Coulthard 9 seasons
Hakkinen 8 and a bit seasons
Prost 6 and 2/3rd seasons (which includes his first year as a driver)
Senna 6 seasons
John Watson 5 seasons + 1 race
Lauda 4 seasons
Hulme 4 seasons
Berger 3 seasons
I think it's good for F1 too. I think McLaren/Red Bull/Ferrari have their two slots locked out for years at a time. Coulthard/Webber/Massa/Alonso and even to an extent Hamilton locking out someone else who could reasonably be there.