Using a 10 speed Shimano 105, do the shifters ever wear out? Should I get a spare just in case?
Many shifters can are used for the whole lifetime of a bike. They are also pretty expensive.
It is also very likely that there wilm be replacement 10-speed shifters available in future, but it is true that Shimano switched to a different cable pull.
Anyway, given the price of the shifter and the lower likelihood of needing to replace it, I would NOT buy a spare just in case. If you were really unlucky, you might be forced to buy also a new rear derailleur in the future. But at that time, if the shifter is worn out, the rear derailleur might well be as well and a new RD is cheaper than an unused shifter in the cupboard.