I watched a youtube video recently looking at the 'progress' in camera tech and it made some good points that relate to this question...
If a lens was considered to be very good / very sharp in f-mount format - bringing in a sharper / better lens in z-mount format doesn't make the original lens worse than it was when it came out. So the question then is - what does that additional sharpness bring to the table:
- photos now in focus where you missed previously - probably technique as there were plenty of pros getting stunning photos with f-mount lenses
- photos now at an 'acceptable level of sharpness' whereas previously we were compromised? - not really, we didn't feel compromised previously and we have decades of photographers producing stunning photos, the additional sharpness now is perhaps a pixel-peeper's dream but in reality probably makes no real difference to how the viewer enjoys the photo - and enjoyment of photo is far more in emotion, story, moment, composition, etc. than directly in sharpness...
So, there is a good question to be asked in what that additional sharpness provides - and then to ask whether it is worth the cost...
f-mount:
14-24 = £1,749
24-70 = £2,099
70-200 = £2,399
total: £6,247
f-mount 2nd-hand (9 or above at WEX)
14-24 = £637
24-70 = £1,009
70-200 = £520 for the older but good model v. £989 for the newer model (both condition 8).
total: £2,166 / £2,625
z-mount:
14-24 = £2,359
24-70 = £2,599
70-200 = £2,499
total: £7,457
£1,210 cheaper to buy f-mount new over z-mount
under 1/3 of the price to buy good
second hand f-mount lenses...
If you can afford it, and the extra sharpness satisfies you more... then I can understand someone buying z-mount lenses to replace f-mount ones - but at a commercial level or for most photographers shooting as their hobby, it is a challenging set of numbers for little additional advantage...
I have been taking photos for 38 years, generally consider myself able to take a reasonable photo - but I am yet to be better than my kit. For me I have a mix of bodies anyway with a D780 / D850 and Z50ii so it makes no sense to upgrade to lenses that will only work on the one body - but even though I might also get a z8 / z9 when the next version comes out, I still think it unlikely that I will upgrade lenses...