They don't have to switch all phones to Android - only the consumer oriented ones. As they prove to be more and more successful, they can try to promote them to enterprises as well - and that's how RIM stays relevant in the future.
Yes they do improve their OS, but like they say "fresh and familiar" which means only their user base would want one, not necessarily everyone else. Most of the people who bought the Torch were already blackberry users, and ~60% of BB users have said that they are not going to buy another BB phone. Their userbase is declining.
RIM is exactly in the position of Nokia, in 2007. It just took longer for them to get here because they were mostly a B2B company, not a consumer company.
But it's already obvious that they are going through same steps and mistakes Nokia did. First they keep the same OS and improve on it, then they build a "new one" (RIM is already rumored to build one for the tablet so it might come to phones too), just like Nokia built Maemo for their MID's and now it's switching it to phones.
After 3 years, Nokia's profits, market share, and stock prices are still declining fast. Analysts are already recommending selling of RIM stock because they dont see a very bright future for them.
Oh, and the failed Ovistore, too. Same story with RIM. They've just improved their Appworld, but it's too late already.
RIM doesn't understand that it doesnt matter if you have a better phone, and it doesnt matter even if you have a better OS, right now. It's all about the ecosystem, and all by themselves they cant create or sustain an ecosystem as the one Apple created by being the ones to transform the industry, or Android created by allowing every other manufacturer to have an OS like iPhone's.
I've shared more thoughts on this here:
http://nextparadigms.com/2010/07/21/5-reasons-why-nokias-ceo-failed-big-time/ and here:
http://nextparadigms.com/2010/08/20/can-rim-succeed-with-a-custom-os-for-blackpad-hint-no/