I've been thinking the same thing for a while - have been relying on ginabot to tell me when some people post - but that's not very practical.
I'm sure lots of people I initially followed have changed what they are posting too (though I'm guilty of that).
As for inactivity, I understand people go through periods of not having the motivation to post, but I suspect the churn on users is quite high, so anyone who is inactive for more than a month might not be coming back.
I'm not against people changing their content. Theoretically, that should expose you to a wider variety of people and thus a net increase in followers, which is good. It's more people posting for the sake of posting and lowering the quality of their blog that annoys me.
I think the churn rate is high too. People are all excited and enthusiastic in the beginning but when they don't see the big bucks and the ideas run dry and they haven't built any relationships, they leave. C'est la vie I guess.