Braaaaptor
Starbucks Enthusiast
Sitting is not good for any vehicle, things don't get lubricated, and I'm not just taking about the engine. A/C for example, the refrigerant IS the lube so if it's not run periodically, o rings and seals tend to fail/dry out/crack. If it's me, I'm not budging to pay anywhere near what they're asking and I'm getting an extended warranty tossed into the deal to CYA.
When I was shopping around for mine I also came across an '18 Ruby Red that looked interesting, but after considering the minor but necessary improvements that they made on the '19-20 trucks vs the '17-18 trucks (looking at you cam phasers and plastic oil pan...) I'd say it's a no brainer to get a '19 unless the dealer already made those fixes and is willing to let it go for a great price.
I'd also me nervous to drive/push a truck that's likely never been up to full operating temperature or driven more than a few miles at a time. like @Rocsteady said, a sitting vehicle likely hasn't been properly lubricating all its gaskets/seals/lines and I'd hate to find that out the first time you go WOT.
Lastly, if re-sale value is something you care about, buying a model year or two newer will impact that value when you trade it in or sell it privately based on KBB value. If you can make the dealer eat this money off the bat then it's worth it but from the sounds of it they must think they have unobtanium on their hands.