@thiggins did some testing/writeup about this a few years ago. He might be able to link to his article.
Newer routers come with a few improvements that can help with coverage:
- Better antenna diversity (when going from 3 to 4 streams, for instance), helps coverage by having more bounced signals and also helps with Beamforming
- Better amps and filters, which might be able to better handle weaker signals with less signal distortion
- An RT-AX86U does offer a few improvements technology-wise over an RT-AC68U. Beamforming can help improve coverage at medium ranges for instance.
- Switching to Wifi6 from Wifi5
- Radio tunings. Radios don't just blast at full power at all time (unlike back in the 802.11g days). There are elaborate calibration files used by the driver that determines the power level used by each stream based on the number of streams, the encoding method, etc... These may get finetuned over time to improve coverage and/or performance.
Now, people shouldn't expect miracles, especially if they are just upgrading from Wifi6 to Wifi6. But when upgrading from a 7 years old router, technical improvements can help with coverage and speed.