>> Nobody relies on estimates from a single satellite alone.Yes they did. From the paper I just cited:
The six main groups that provide satellite-altimetry-based GMSL estimates (AVISO/CNES, SL_cci/ESA, University of Colorado, CSIRO, NASA/GSFC, NOAA) use 1 Hz altimetry measurements from the T/P, Jason-1, Jason-2 and Jason-3 missions from 1993 to 2018 (1993–2015 for SL_cci/ESA). The differences among the GMSL estimates from several groups arise from data editing, from differences in the geophysical corrections and from differences in the used method to spatially average individual measurements during the orbital cycles
The apparently independent time series are actually computed from the same raw data sources, as there isn't an abundance of redundant satellites measuring sea level.
But isn't this goalpost shifting? At first you said I was wrong that uncertainty has been the same size as the measurements in the past, citing the IPCC as proof which of course doesn't mention any of this, saying only that they have high confidence in these numbers (the IPCC is not trustworthy). Then it became that the error isn't big enough for you, and nobody relied on a single satellite. Now I show that they did indeed rely on a single satellite.
The core issue here remains the same: how do they know they're getting it right now? A calibration error so large it invalidated their entire time-series lasted for the entire lifetime of the TOPEX-1 mission, and it then took 20 years for the problem to actually be detected and corrected for. They've only been measuring sea level with satellites for 30 years, and the discrepancy between tide gauges and satellites has never been properly reconciled even though they theoretically measure the same thing.