Is there a bright line between cost reduction and planned obsolescence?
Obviously a small unreplaceable battery is not a good example for that discussion.
I had an interesting situation where we had failure of a Thule bike trailer wheel and could see where the connection-to-the-trailer design had changed from an earlier version (from the company that Thule bought). The wheel functioned the same, but you could see a clear difference which fully explained the failure. I expect it was a cost optimisation, and we only encountered the failure because we used it very heavily.
Edit: they also failed to honour their warranty commitments, but that was secondary.
Going out of your way to make sure the gauge doesn't work after the battery is replaced surely is.
I think there is: It is the line between "not spending extra money to make sure it works" and "spending extra money to make sure it won't work".
There is a related problem with warranty: an inferior third-party replacement part may cause damage to higher-quality original parts. There is a line here between "making sure you don't have to deal with follow-up damage caused by inferior parts" and "preventing the use of inferior parts". This is a bit more blurry because most cases won't be clear-cut, and dealing with them will be a burden on the original manufacturer.
I think it is important that we reward the nice players as much as we punish the bad ones. A blanket "all companies bad" just means that no company has an incentive to be anything less than bad.