Microsoft blocks Windows 10 update over Boot Camp driver issue
Owners of older Macs or those with out-of-date versions of Boot Camp are being blocked from updating Windows 10 to version 1903.

Users of affected Mac devices that use Boot Camp attempting the update will see an error message warning about a problem with the Mac HAL driver 'machaldriver.sys' on their Windows 10 installation. "Your PC has a driver or service that isn't ready for this version of Windows 10," the message reads.
"No action is needed. Windows Update will offer this version of Windows 10 automatically once the issue has been resolved," it concludes. Microsoft advises it is working on a resolution and estimate it will be available in late July.
According to a Microsoft support page found by Windows Latest, the problem occurs with Macs released before 2012, as well as newer Mac models that use older Boot Camp versions or have Windows Support Software drivers with a compatibility hold in place. More specifically, the problem lies with the identified driver dated for September 24, 2011 or older in the Windows\system32\drivers directory.

Microsoft's error message when updating using an incompatible Boot Camp driver
As a workaround, Microsoft advises updating the Boot Camp Windows Support Software drivers could replace the driver in question and enable an update to go through. Microsoft suggests users can check for Windows Support Software updates in the Mac App Store and also links to an Apple support page dealing with installing the Windows Support Software itself.
This is not the first time Microsoft has prevented an update for Windows 10 based on software connected to Apple. In November, Microsoft stopped users of Apple's iCloud for Windows tool from updating to 1809, due to issues with updating and synchronizing shared albums.

Users of affected Mac devices that use Boot Camp attempting the update will see an error message warning about a problem with the Mac HAL driver 'machaldriver.sys' on their Windows 10 installation. "Your PC has a driver or service that isn't ready for this version of Windows 10," the message reads.
"No action is needed. Windows Update will offer this version of Windows 10 automatically once the issue has been resolved," it concludes. Microsoft advises it is working on a resolution and estimate it will be available in late July.
According to a Microsoft support page found by Windows Latest, the problem occurs with Macs released before 2012, as well as newer Mac models that use older Boot Camp versions or have Windows Support Software drivers with a compatibility hold in place. More specifically, the problem lies with the identified driver dated for September 24, 2011 or older in the Windows\system32\drivers directory.

Microsoft's error message when updating using an incompatible Boot Camp driver
As a workaround, Microsoft advises updating the Boot Camp Windows Support Software drivers could replace the driver in question and enable an update to go through. Microsoft suggests users can check for Windows Support Software updates in the Mac App Store and also links to an Apple support page dealing with installing the Windows Support Software itself.
This is not the first time Microsoft has prevented an update for Windows 10 based on software connected to Apple. In November, Microsoft stopped users of Apple's iCloud for Windows tool from updating to 1809, due to issues with updating and synchronizing shared albums.
Comments
I'd ignore all advice above and use the 'if it ain't broke' philosophy for now.
Happily using a 2011 iMac on my desktop. If now doesn’t get the latest macOS, but continues to receive updates. The fact that it is completely functional and runs VMs for software development, and has never required me to wipe it or re-install it like I did a million times on Windows, is a testament to Apple.
Not sure if it helps, but you might try these (if you aren't already). For my 2018 mini w/ eGPU, it won't work 'out of the box' so I have to install these drivers (after uninstalling the stock ones a couple times... the couple of times being key!) I'm hoping next time I rebuild with 1903, things are more sorted out and it works w/o as much tinkering. That said, the people who did it pre-1809 or with MBPs seem to have had to do a LOT MORE tinkering (so I should be happy).
Yeah, though I think the whole eco-system is moving faster these days, so to people who are used to how it worked decades ago, it kind of feels like that.
Why not compare the top of the line 2019 iMac to the Top of the Line 2011 iMac? or 2018 Mac mini vs the 2011 Mac mini? They are very different class computers
The 2018 Mac mini will out perform the top i7 2011 iMac's in most specs except GPU probably -
using GeekBench as an example 2018 Mac mini with i7 gets around around 5700 single core, and 24000 multi-core
the 2011 iMac top-end i7 gets around a 3400-3600 single, with the top scores showing 11000-12000
Disk performance is many times faster in the 2018 Mac mini - high-end SSD in new vs traditional HDD - or a much older model SSD was still SATA speed limited , as is wifi & bluetooth performance better on the new hardware.
in GPU performance it looks like the Radeon HD 6970M is about 34% faster on average than the Intel Integrated 630 video - I'm honestly surprised it's that close - but there are GPU elements that the Intel 630 is faster at, so it's not a complete win for the 2011 iMac, and throw a cheap RX 580 in an eGPU case can get much faster performance (something I will actually be doing some app tests today or tomorrow with)