I use Pentax and NiKon, the old Pentax lenses still fit the new DSLR but some have a tendency to jam on the electric contacts, not got any old Nikon.
Also the old lenes are big and heavy, the DSLR are not full frame so the new lenes are smaller. Also the old lenes do not auto focus, and in some cases will not auto shut down to the aperture selected, and getting light levels correct is a real pain.
With the Tamron lens on the Nikon I can simply point and shoot, with the old 400 mm lens on Pentax it is a lot more work to get same shot, so although the Nikon is only 270 mm zoomed in because 16k rather that 10k of Pentax the result with cropping is the same.
I have dirt on Pentax sensor due to changing lenes, so Nikon the lens never comes off. 18 -270 mm it does not need to come off.