For the price I’m hoping I can go quite a bit longer on the Mac, so that leads me too my question of how long has yours lasted you and have you noticed a significant slow down over the years with new programs and updates coming out for software?
Macs tend to last pretty long. I will be upgrading to the 2019 iMac, coming from a 2011 MBP (which of course was slower than the iMacs at that time), which still runs all newer versions of software I've been using over this period of time. Strangely, it even runs Lightroom and Adobe stuff (CS 6
) fast "enough", while many people complain about how slow LR is (ok, exports take their time, but editing is definitely not sluggy). What is slower of course are new functions the programmers add (like HDR stuff or panorama stitching), that e.g. utilize the GPU or benefit from more CPU cores, but the basic editing functions are perfectly usable on this "old" machine. Of course there are exceptions, like On1 Photo RAW, which was pretty unusable in the 2017 version, but has been improved a lot-and it was also slow on newer machines. And surely I never tried editing 4K video on the MBP.
I've always heard that a Windows machine will get slower over time as you install more software, because the system becomes increasingly inefficient (a clean install every 3 years or so helps, apparently)-mind, that's only what I've heard. This is definitely not true for Mac OS. What is annoying with Mac OS is that over the past years they have been pushing out a new OS version every year, with shiny new functions I really don't need in most cases, instead of making the software more reliable and efficient (my personal opinion!!, and experience of friends and coworkers). And they tend to force their programmers to need to adapt the software to new OS versions, which of course is good from a security standpoint, but sucks if a program stops working you have been relying on for quite a while, just because it's no longer updated.
This is why I am running two partitions on my current Mac, one with OS 10.11 (for running Office 2008, which I need for my job, and Adobe CS 6), and one with 10.12, because the latest version of Capture One will not work with older OS versions... sorry for this little rant. I guess in spite of these "annoyances" (which arise from me e.g. not willing to pay the Adobe "tax", so also a personal problem) Mac OS overall is still more reliable than Windows, and luckily I can install older OS versions in virtual machines, which will let me run my old software on my new machine, too.
Long story short, your new iMac should be good for at least 6-8 years, even if Apple comes out with a new hardware architecture in the next 1-2 years. They have done that twice in the past and made sure the transition was manageable for their user base. You already have done the wise thing and ordered an SSD-in my 2011 MBP I could install one myself, and it has sped up the workflow extremely. Now put some RAM into that iMac and you're good to go. Have fun with your new computer!