My point of view :-
DDR4 and PCIe 3 are definitely not dead end platforms,, they will be good for few years to come, so its not a "bottleneck" situation for buying dd4 or pcie3
Additionally there will always be something better and new, if if in future there is Zen4 + AM5 system, AMD zen5 will be even better and zen 6 will be even better and so on and so on.
i personally would try to go close to 25 to 30% cpu single core performance difference before upgrade..... that's roughly about skipping 1 or may be 2 intel generations in the past,,, like going 7th to --> 9th or even 10th gen.... and matching up to the bugdet and new motherboard.....
this is because i am a casual gamer,, so 5-10% difference would not really help me ... and i do not have any heavy multicore work
Please note :-
for my personal PC, Currently i am deciding to upgrade from 7700 non-K to 10600K/ 3700X ,,, bugdet around 450- 500 ... AMD 4000 launch has confused me as well ...