Bmw M once was very special with its bespoke NA engine,manual gear box,RWD and light weight frame.but now,it gave up all these exclusivity to follow Audi and Mercedes's formula in order to accomplish a faster acceleration in straight line.This is sad and why the new M5 is regarded as a underwhelming one among competitors.On the contrary,Bmw M5 in the E39 and E60 era is outstanding.
You praise the E39 and E60 M5's as outstanding, even though they don't meet your own criteria for what is very special?
The E39 M5 was heavier than the F90, the E60 M5's are the same weight as the F90 - Not that M5's have
ever been classed as lightweight. They are normally the heaviest 5-series' you can buy.
The E39's S62 V8 is a tuned M62, the M62 appeared in many models - some of which weren't even BMW's (Range Rover, Ascari, Bentley, Morgan). This isn't really any different to how BMW use the S63 engine today. Granted the S85 in the E60 was a fairly unique beast.
The E60 M5 also didn't initially come with a conventional manual transmission either, it was only late on that they introduced a 6MT, and guess what, it wasn't very well received.
So, I think you are looking at the past through rose-tinted glasses.
As for your other points, the move away from naturally aspirated engines has nothing to do with BMW giving them up to follow what Audi and Mercedes do... it's a direct result of legislation imposed on BMW for emissions and consumption reasons.
BMW M also hasn't given up on manual gearboxes, you can still get manuals in the M2, the M3 and the M4. The reason why they've been dropped in the bigger M cars is because the take rates are low-to-non-existent... it might make the M5 seem less special to you, but the people going and putting their money down, do not want manual gearboxes.
If you find the F90 underwhelming then okay, that is your opinion... but the reasons you've given why, don't make much sense.