I think the notion that you can get any 600/4 MkI for $6000 is misguided. You can find them for that cheap...but when you look at the condition of the versions that people are selling for that low of a price, they are NOT in the greatest of condition. Nicks, scratches, enamel discoloration, missing or broken accessories, etc. I was just in the market for these lenses, and I searched every store online, including eBay and Craigs List, before finally buying a brand new EF 600 f/4 L II from Vistek in Canada (the USD price was $10,865, plus $67 shipping and a 1.5% currency exchange fee...couldn't freaking pass that deal up!!)
In my searches, however, I found that an EF 600mm f/4 L Mark I lens, in good condition, with all accessories in good, working condition, including the hard case, was closer to $9000 than $6000, and in a couple instances where the whole kit was in perfect condition, not even a nick in the enamel, AS SOLD prices on eBay were ~$9800. Assuming the condition and completeness of the kit matters to you, the difference between an old Mark I and a new Mark II is not $6000. That would be the difference between a brand spankin new, list price Mark II and a fairly beat up Mark I. The difference between a sale price Mark II and a great condition Mark II is maybe $2000 to $2500.
In that respect, if you own a Mark I, and you've kept it in good condition, have all the accessories and the hard case...your "upgrade price" is likely to be far less than $6000, possibly as little as $2000. For that price, the reduction in weight, better AF, double the IS capability, superior IQ, and a warranty (!!)...well, it's well worth it!