A new algorithm may be incompatible for ASICs. It isn't easy for developers to just make a fork and make all the users and miners to switch to a new client. It doesn't benefit those with big ASIC farms since all their hardware would be wasted.
And that is really bad? If the new algorithm is incompatible with asics, maybe many of the users (who are far from mining because of the dificulty and costs) will start to mine themselves for the stability and safety of the network...
Yes. For people who invested a lot of money into mining, they don't have any chances of getting ROI. If the new algorithm can be CPU mined effectively, botnet mining would resurface. Botnet mining are most probably stopped now since the difficulty is so high.
I'm guessing they are still quite present, since they cost nothing for the operator. And they are most likely very profitable with X11 or X13 or whatever.