My understanding is that (at least for Himawari-8) it is geostationary at a high enough altitude (apogee and perigee) that it should be good for at least a few

My understanding is that (at least for Himawari-8) it is geostationary at a high enough altitude (apogee and perigee) that it should be good for at least a few decades - apparently you actively have to boost geostationary satellites out to a retirement orbit so they don't crowd up the prime real estate. That said, the imaging sensors have a limited lifetime (the satellite platform is designed to be good for ~15 years but the sensor package limits the mission lifetime to 8 years).