Why indeed? I regard the ‘need’ for mirrorless to be more an issue of manufacturer desire …like energy smart meters and hybrid motors.
Mirrorless is on the whole a series of compromises.
The evf on the Sony A1 is the best I’ve seen…but it’s still not the same as a DSLR viewfinder. Good enough but compromise 1
In common with other mirrorless bodies (except this Z9) the bodies are small. Not sure why when hands haven’t changed size. Compromise 2
The lenses used with most mirrorless systems are still heavy DSLR versions. This means in use, mirrorless + lens is not much different to DSLR + lens. Compromise 3 (Only Fuji X system has light weight bodies AND lenses.)
Battery life is less than half of a DSLR. Compromise 4
The responsiveness (wake up, focus acquisition) of many mirrorless bodies is slower than a DSLR. Compromise 5
The Sony A1 is however very fast to focus and retain lock under moving subject situations.
It’s mechanical shutter is very silent….that’s even without going to silent mode!
So there is some hope things might improve in the areas where I feel we are being required to compromise.
Mybiggest disappointment with mirrorless is the meagre improvement in noise at relatively low iso levels. Not everyone wants 250,000iso. And if we did, there would be realistic expectations of some serious noise.
But in the more typical overcast UK light, ISOs upto 6,000 are often necessary to allow faster shutter speeds and narrow apertures as the subject dictates.
For me the only reason I would contemplate changing systems (again) would be if a manufacturer manages to give iso 400 noise levels upto iso 6000. It would save a lot of faffing about using noise reduction software.
I agree with all of your points except the first one about the OVF/EVF part.
With DSLR there is mirror blackout which is analogous to the blackout in mirrorless when it reads out the sensor.
The battery issue is a common problem with all electronic cameras. Film cameras only needed power initially for the meter, then meter and film advance, then finally for meter, film advance and autofocus plus any TFT for menus.
Still learning after all these years!