Why would you want to keep every email you've ever received? Yet isn't that the principle behind Gmail?
It's far from unreasonable to want to keep all your messages. Sure, you'll never need most of them, but you never know when you might want to pull an obscure text that somebody else remembers different, or has an address on it you didn't think you'd need again. When I switched to Android from Windows Mobile, I invested a lot of time in transferring all my messages from my old phone, and now have 1569 texts. I don't think Android shouldn't be able to handle that number, but I do regular back-ups, because I have heard of this problem. It's something Google should rectify.
OP, for backup, I recommend "SMS Backup and Restore". It's free, works perfectly, and stores the messages in an easy-to-read XML file, so you can transfer it to your computer and edit it manually if you like. It might be worth downloading it and running a backup, see if the messages are maybe still on the phone, just not showing up for some reason. Run the backup, then look for the file sms.xml in /sdcard/SMSBackupRestore/. If your messages aren't in that, then they're probably gone for good