Nope, not really. Developers will take a while to take advantage of that machine and just like Windows aspect will still be around so will the Intel Mac. This is not going to be like PowerPC to Intel as back then there was no reason to keep powerPC as everyone wanted intel as that architecture was (and still is) the standard.
If/when Apple jumps to ARM they will do the transition fast but software will take a while to catch up and I'm sure what you buy now will be good for 5 years if not longer.
Also, you never want to get first version of a product especially like this. Not only that it might have some bugs but the 2nd generation is always 100000x better as the follow up always overshadow the first gen.
So no, iMac now (the 2020 model) is probably the best purchase you can get (pending to see what the reveal is of course)
If/when Apple jumps to ARM they will do the transition fast but software will take a while to catch up and I'm sure what you buy now will be good for 5 years if not longer.
Also, you never want to get first version of a product especially like this. Not only that it might have some bugs but the 2nd generation is always 100000x better as the follow up always overshadow the first gen.
So no, iMac now (the 2020 model) is probably the best purchase you can get (pending to see what the reveal is of course)
I understand what you mean but there is a risk of having a machine born "old" and with a limited useful life (in terms of future software compatibility).