In the past, I would have not disagreed with you on avoiding new problems firmware might bring up. But in recent memory (last few years), I can't think of 1 situation where updating the firmware in a TV, bluray, or streaming box has caused an issue.
However, NOT updating HAS certainly caused issues.

With 4K in such a messy state, if you are NOT upgrading the TV firmware to the latest, you are doing your customer a huge disservice. You are most likely causing the TV NOT to able to decode things like Dolby Vision or HDR10+. Or have the correct LUT for SDR & HDR content.
It would be unforgivable to sell a customer a flagship model TV, then cripple its performance due to ignorance.