There are a couple of things here.
1. Indeed, a change of algo would make Bitcoin safe of BCash movements, especially as they have proven already to be hostile against Bitcoin.
2. A change of algo should also be thought in a way the power consumption is decreased, since there is always bad publicity about this and more power means more expensive mining, which may be a problem when block rewards will get really small.
3. A change in algo may make many big miners angry against Bitcoin and politically it may not be a good move to make them go into BCash boat.
4. A fork is usually advised to be avoided as much as possible since it is perceived as lack of stability.
5. One of Bitcoin's main powers is the huge hash power supporting the blockchain. If that starts again from zero there are many altcoins (including BCash) that will try to claim the crown.

All in all, this needs indeed quite a lot of thinking and all the alternatives weighted well, since if not done properly it may do much more bad than good.
Right now I don't have any idea how this could be done better than we have - with all the problems, I know.