First: I have 2x30" screens. I don't want to go down.
Second: I can't put multiple HDDs and SSDs in an iMac.
The new system replaces a 24" iMac I bought in 2008. It made it for 5 years; the only upgrades were extra RAM when new and swapping the HDD for an SSD about 2.5 years into its lifetime.
I don't know how long I'll keep this one. Under Australia's small business equipment rules, it's already fully depreciated. So there's no accounting purpose to hold onto it if I don't want to.