Why you shouldn't 'invest' into an AM5 platform now
First off all buying hardware isn't actually an investment if you don't buy it to earn more money back. If you are buying something that will just depreciate in value without making you any money it isn't actually an investment. It's just you spending money for consumption.
One general issue with zen4 is that it comes with what used to be HEDT pricing without delivering HEDT features such as quad channel ram or 32+ pci express lanes directly from the CPU. You are still limited to at most 16 cores and you cannot use 128 GiB of DDR5 without dropping down to
just 4000 MT/s which is well below what good DDR4 supports.
Raptor lake support 40% higher memory bandwidth when 128 GiB DDR5 is used.
Some people have suggested buying a low-end zen4 CPU now in order to upgrade later to a better CPU on the same platform but that really doesn't make sense. It makes much more sense to wait until there is actually a CPU you want to buy and then have much better motherboard options to choose from.
The current am4 motherboard chipset seems to be bottlenecked by a single gen4 x4 connection, AMD and motherboard vendors have been awfully quiet about this which is explained by them having something to hide (same with pluton).
anandtech.com/show/17585/amd-zen-4-ryzen-9-7950x-and-ryzen-5-7600x-review-retaking-the-high-end/4
While zen5 is very likely to work with AM5 you are probably going to miss out if you use an old X670E motherboard rather than the newer X770E or whatever. You might also realize that you want to use intel instead and then you end up having to not buy your preferred products or give up on your AM5 'investment'.
Since zen4 has a bad memory controller there is no way to test the signal integrity of AM5 boards when it comes to potential DDR5 overclocking (for zen5, etc) since the CPU imc limit you long before signal integrity of the motherboard becomes a factor.
One downside with using old hardware besides probably missing out on performance/features is that you then need to disassemble your old system, doesn't mean that you shouldn't do it but often it makes more sense just to build something new, you might want a second computer for example. If you for example buy a cheap DDR4 board to re-use your old DDR4 you will need to take that DDR4 away from your old system which makes that computer unusable until you find replacement ram, upgrading to DDR5 later is an option but then you also need to replace the motherboard so you might aswell buy good DDR5 right now (it isn't to expensive) and get that extra performance right away.