Maybe over time having an imbalance in the springs causes the shocks to compensate / wear at different rates? I think all manufacturers specify replacing them in pairs.
I haven't done the shocks on the MKIII Octy yet but I did replace both springs at a cost of €170 including new strut top mounts, pinch bolts and drop links back in Feb (2025). Gretta failed the NCT on a shock imbalance during her taxi days in Nov 2016 with a mere 195,860 kilos on her clock. I got Aiden to replace the shocks, coil springs, strut (top mount) bearings and strut dust covers which including the PS wishbone cost €436 for parts and labour and another €40 for Collin to set the tracking on his then brand new machine. The wishbone lasted until she failed the NCT in May 2023 (376,637 kilos) when I replaced it myself along with brand new main dealer bolts (one bolt broke during removal), a pair of track rod ends and the PS ball joint at a cost of €162. The shocks are still in her but she broke one of the springs (a few weeks after seeing the MKIII get a new pair of springs) back in April (2025) so got a new pair of springs, top mounts, dust covers, pinch bolts and drop links which I fitted myself at a cost of €131. I didn't bother doing the shocks on that occasion as they both met manufacturer specifications and she might be replaced with the MKIII next year depending on EV grant availability. The MKIII shocks also tested within manufacturer specifications when I done her springs so I reused them, the rationale being that she might be scrapped next year.