Let's hope that the selling-out of Twitter will give a nudge to its open decentralized competitors... I'm not holding my breath, but who knows ? I still dream of an XMPP Twitter...
I don't think that Twitter's effort to monetize is directly equal to selling out. If the service's quality degrades significantly (too many pushy ads and buy buttons) then, I'd say it sold out. So far, I've found the ads fairly relevant and not too frequent.
I may be oversensitive to spam (I must admit that my love of ad-blocking software in all sorts of systems borders on mania) but the very idea that I now have to be wary of Sponsored Tweets that require an effort to unsee has already started to spoil the experience.