MultiBit got progressively slower after adding more receive addresses At first, MultiBit was all snappy. Over the course of about 4 weeks I added ~120 more receive addresses. Now, MultiBit is very sluggish, grinding the hard-drive excessively (sometimes for minutes), especially when sending payments or when exiting the program. Today, it displayed a message in the status bar that it couldn't open the wallet and that it will use some rolling backup. After that, it resynchronized for about an hour.

I worry that if things continue this way I will be faced with catastrophic data and money loss.

Do you have any hints on why this might happen or whether I could migrate to another more reliable wallet software which could import all my addresses?
It seems very unlikely that a number of addresses as small as 120 could make a significant difference in performance. I would guess there is some other problem that just happens to coincide with you adding addresses.

The inability to open the wallet, and the excessive disk I/O, is particularly troubling. One possible explanation is that the hard drive is failing - the slow I/O being caused by repeatedly retrying failed operations. I would start by backing up all your files and especially your wallet (which you do regularly anyway - right?). If that fails because there are files that the backup software cannot read (without a good reason, e.g. they are locked by another process), or it reports I/O errors, then that would clearly indicate a failing drive that needs to be replaced.

(Generally, if you have good, regular backups of your wallet files, you are fairly well protected against "catastrophic money loss".)