Intel part availability
It's all very well Dell saying their hardware models will be available for 5 years, but Intel CPUs don't have that lifespan. You might go to dell in 3 years time and say "I'd like the servers we got before" and they will have to say "We can do the board but the Xeon E5678 parts you paid for then aren't available no more". If the replacements fit into the existing CPU socket and are compatible with the chipset (which implies qualification) then you will end up with a cluster that is only weakly homogenous; the difference in performance will lead to quirks: race conditions etc, and may need new configs to deal with their ability to do more work in less time.
If the CPUs aren't compatible with the old chipsets -and have you tried to get a recent Xeon into a 5 year old box?- then they're going to have to say "sorry, we lied". Or they buy up a roomful of the old parts before they go off the intel price list, which given Dell's attitude to inventory is unlikely.
All x86-based HPC server vendors have this problem; it's why things like ARM designs may have an edge. If the foundry can still fab in the same process, that Core design you bought off ARM will still work.