I love very much my n900.
It makes me sad that the emmc lives only a limited time.

Is it possible to replace an emmc once when it is worn out?

no dont think its possible. but I highly doubt it would stop working. Think of a memory stick, i have an old memory stick 2GB which is 3 years old. I use it regularly and that hasnt worn out. then how can the 32gb on N900 would stop working. flash memory can take so much beating before it stops working.

ps I now own a portable hardrive dont think im an oldy

I think there is a difference between a normal usb stick and the memory of a linux device.

You put your data once on a USB-Stick. Then you read it many times.

A Swap is written many times per second.

I would suggest you do a little research on contemporary SSD's - over USB the eMMC in the N900 would require writing data to it for roughly 10 solid years non stop before you see any kind of wear issue. You're far more likely to have your N900 die of other causes long before then.