It is always a good time to by a Tesla if you want to drive the best car available. You are welcome to wait for future improvements, but you are fooling yourself. There will always be future improvements to wait for and while you wait you will be driving a markedly inferior car made by some other company.
Is that really the full picture, though?
One reason why car companies have model years and lifespans for cars is that they actually mimick the average new car purchase cycle as well. From an individual customer perspective it makes sense. Even companies like Apple have predictable iPhone releases and that is a much smaller investment. You get all the benefits of that year, instead of TouchID in June and then better camera in December... Imagine if buying an iPhone was as difficult as buying a Tesla?
With Tesla your car is often obsolete before you even receive it, especially if you live abroad, because of the piecemeal approach. There is no way around this, no timing that really can help you avoid that. With other companies you have a much higher chance of planning your purchases to coincide with the cycle, but not with Tesla as everything is always in flux.
So it is not just about constant innovation with Tesla, it is also about their tendency to have a very immediate reaction in the end product, instead of collecting and grouping changes for the customer's benefit. Last time Tesla had a little bit of that was the D-A launch. Since then everything has come (and gone) piecemeal. And it isn't just every 12-18 months. Even big companies have model years and a small mid-year change. But with Tesla it is every 1-3 months that things change, often dramatically. Facelift in Q2, P100D in Q3, AP2 in Q4, 100D/removal of interior designs and ventilated seats in Q1 etc. and more...
What this results in is understandably apprehension and disappointment on the part of the customers - some of which could be avoided by grouping these changes into one more often. It might also help avoid some releases that have gone out too fast (e.g. P90DL Countergate issues) with apparently too little testing.
I think Tesla is being too aggressive at the moment for their own good. If they really grouped their changes into 12-18 month bundles, that would be excellent. But instead they are changing everything all the time, making for quite a stressful purchase process. I can see why some might gladly swap that for e.g. the German premium way of doing things, once their EVs become available.