I'm getting one as soon as it's available, along with the new Pro Display XDR, but I'm going to try renting instead of buying. There are a couple of companies in the UK who offer this, and I'm thinking it's a better option than buying as you can return the equipment at the end of the lease (two or three years) and swap it for the latest model, including the display. I can't really think of a downside to this as I never sell my old computers and just leave them lying around or give them away.
The downsides depend upon context and several factors.
If you have the previous generation 'lying around' then you have 'cold' backups or possibly a small farm to throw peak load batch jobs to. If you lease then they goes away at the end of a lease.
Also depends upon the rental rate , business tax rate (and deduction schedule for capital equipment costs. ), and available borrowing rate (and/or available investment cash).
The rental companies have their borrowing rate and a profit margin stuffed into their pricing. If can match or beat their effective rate with a lower borrowing rate then can acquire the backups/farm alluded to above at much lower cost.
It can also have a downside if spending more if the margins and/revenue flow aren't substantive enough cover this. If a $6K screen brings
zero increase in real revenue there is a downside. For some narrow workload areas it can make a difference. However, for a large number of ones it won't; even when scoped down to the classic Mac Pro user space. Additionally, if Apple doesn't have a evolutionary better screen ready in 2-3 years , then it is a total downside. You'd likely have to "buy out' the XDR at the end of the lease and that will be higher than just paying for in the first place.
There are rumors that Apple is going to roll out microLED displays and then next discrete monitor could be fantastic in several years. I wouldn't bet the farm on Apple delivering on that in 2-3 year cycle. This Mac Pro 2019 is years late of where it should have been. The iMac Pro will likely stumble into 2020. Probably no microLED on the Watch this year. For a substantive new Mac Pro in 2-3 years there is a decent chance given Apple could make some upgrades to the motherboard and release. Apple sat on the 30" and Thunderbolt Display Docking station for way longer than 3 years. ( 6 and 8 years respectively ). if they revised this in 4 years that would be them greatly picking up in speed. ( something that was more mainstream 3 years is closer to their cycle, but this isn't mainstream at all. They'll have to 'top' this to directly replace it. ). Minus a major change in backlght there isn't much of a "major change" Apple can make in the display to something that is going to be "big leap" better. ( I wouldn't bet on 'more pixels' solely driving them forward on this product in the intermediate term. )
The XDR i'd wait to see what the quality is once at volume production. It is also a "nobody else is using" backlight for which longevity isn't tested. The downside here is get an early long term lease on a display where the hype was higher than the long term performance. The exit terms on the lease could be a substantive downside. On a relatively short term lease ( months to 'try out' on several projects) could have some upside even though it is a bit more money before buying.
