I've heard similar things said about the new Royal Enfield motorcycles here in India.

The redesigned J-series engines on their new motorcycles are smoother and I've heard some people wax nostalgic about the old thumpers and say that the new ones "lack character". I've even heard people speak fondly of the the UCE engines (introduced in the early 2000s) which were hated for a similar "lack of character" when they replaced the older engines (which were originally designed back in the 50s I think).

This! I don't know those bikes but here is my hypothesis: older engines required more experience to be fully exploited, as they had 'holes', that it to say revving ranges or global state leading to unsatisfying performance (mainly due to tire, frame, brakes and carburetor's limits/defects): one had to learn, by practicing, how to avoid/circumvent such problems.

They also are accustomed to the effects of sub-optimal or economical designs, the main example being vibrations, and learnt to like them. "If it doesn't vibrate hard, stinks, pours oil, yells... it is cannot be a true bike!"

Modern bikes (especially electric) are way less quirks-plagued, more 'linear', easy to exploit and their performances (at equivalent 'cost', inflation-adjusted) are way better on all accounts (grip, brakes, flexibility/driveability, acceleration, max speed, reliability...).