Electronics in high-radiation environments benefit from a large feature size with regard to SEU reduction, but you're correct that the larger parts degrade faster in such environments, so they've created "rad-hard" components to mitigate that issue.
It's interesting to me that triple-voting wasn't as necessary on the older (rad-hard) processors. Every foundry in the world is steering toward CPUs with smaller and smaller feature sizes, because they are faster and consume less power, but the (very small) market for space-based processors wants large feature sizes. Because those aren't available anymore, TMR is the work-around.
https://en.wikipedia.org/wiki/Radiation_hardening
It's interesting to me that triple-voting wasn't as necessary on the older (rad-hard) processors. Every foundry in the world is steering toward CPUs with smaller and smaller feature sizes, because they are faster and consume less power, but the (very small) market for space-based processors wants large feature sizes. Because those aren't available anymore, TMR is the work-around.
https://en.wikipedia.org/wiki/IBM_RAD6000
https://en.wikipedia.org/wiki/RAD750
Most modern space processing systems use a combination of rad-hard CPUs and TMR.