The 450, Roadster and original Forfour use the same tactile switches on the key PCB.
As with most things these days, they are probably the cheapest parts they could find for the job.
They are low quality, have no internal dust protection and are soldered in with lead free solder.
So, they either stop working internally or the lead free solder cracks and eventually, the button drops off.
Replacement is a fairly easy task if you have done soldering before. The parts are small but only have 2 solder points.
Rub a hot soldering iron over the contacts where the switch was, then flux both contacts and the switch connections.
Flux is a mild acid that cleans the connections which makes the solder joint better and allows the solder to flow.
Place the new switch in position. I am using better quality replacements from SmartMods.co.uk.
They have a more positiving switching feel, rated for more clicks and they have an internal dust membrane.
Apply a small amount of solder to the top of your soldering iron and quickly dab the connection point.
Repeat on the other end of the switch. You'll have to initially hold the switch in place with tweezers.
Test the key works and reassemble the key fob. The SmartMods switches are the same dimensions as the originals
If you find that pushing the rubber buttons is too easy and the new switches are too sensitive you can open the key
again, take a sharp blade and remove some of the rubber X from the back of the rubber buttons.