The difficulty is not in migrating to a new service when the old one dies; it's in transferring all your old data across to the new one and updating all your other tools that reference it.
With Reader, (apart from historical read/unread/starred/etc) this is as simple as downloading an OPML file and uploading it somewhere else.
With Gmail, it's more tricky. Migrating emails is fairly trivial, migrating the address itself though (unless you own the domain) involves updating all your contacts and changing your settings for every site you use on the internet (auto mail forwarding of course makes this easier).
With Notes, I'm not sure there's a standardised format for exporting/importing from one service to another. Certainly, with different features between each product a lot of information would be lost when doing so.
If you're fine with that and/or don't keep really important notes in the service then it's not really an issue. If you want to ensure they stick around as long as possible, then you need to pick a service that is committed, or one that you control yourself.