You actually bring up a very valid point....your thought process was EXACTLY what I went through in my mind! Something that could be very important to add to this discussion.
A bit of a back story, my Mooney is a 1968 with just under 3,800 TTAF, and 510 SMOH and 510 SNEW on the 3 Blade Prop. She flies like a champ and burns about 9.5 Gal/hour as we all know. I picked it up for a decent price just under 2 year ago, and so far *knock on wood* it hasn't been too bad. Worst expense so far was a failed DG that needed replaced and the poor Vaccum regulator from 1968 gave out on me last summer. (Less than $1600 combined)
As I started working on my IFR training a few thoughts crossed my mind. I loved the Mooney, from the first day I flew it. But the long-haul trips got hard to manage without auto-pilot. No heading bug etc also made it fun to remember proper paths. (Granted I learned a LOT doing my IFR training BEFORE the panel upgrades.) But I started to look at options and there were two decisions. 1.) Upgrade my Mooney, make it mine, and know that I'm never going to get it back. 2.) Find something new and ditch the mooney.
So I set out, looking at #2 first.
The used market, for a M20C, without auto-pilot etc was horrible. I don't necessarily have the resources to "keep it" while I buy a new plane. So I talked with a few people that take trades and DAMN did they low-ball me. Then I started to compile the list of what I wanted. At a minimum I set myself to need, Auto Pilot, Glass Panel, ADS-In/Out with Traffic/Weather displays, XM Radio/Weather. (Love it from my 496), and less than 750 SMOH. I didn't want to jump up huge in fuel consumption either as then my per-hour costs go through the roof as all of my local places are quite high with 100LL. The SMOH limit was to help "plan" for an overhaul of the engine if needed.
All of this put me into the $145,000+ range, with "trade" offers of less than $36k for my Mooney. That's a lot of Money. I do fly about 100 hours a year or so, but not much more. I then turned to insurance, and some of the birds I was looking at would have doubled or tripled my insurance due to the costs etc. I also started to look at the annual costs. My M20C with the Johnson Bar Gear and Hydraulic flaps gets the job done, and my annual was a very tidy little process. Looking at a new plane that would have gone up.
Compare that to option #1
Well, if I do the upgrades, I know the history of my plane. I've learned its quirks, I know what it likes and what sounds normal. I also know the history of it and how much the prior owner cared for it. With a low time Engine I'm already well on my way of saving for the future overhauls etc. Putting in the panel, was ~ $50K out the door, putting my total "buy price" for the Mooney to around $100k. Is it worth 100k? Nope, never will be. But every aircraft is going to have to get ADS-B eventually and if you take care of your plane it will still go there. For me, financially it was easier to swing an add to the current plane in cash, rather than making a sizable increase in airplane payment. I know the upcoming things, the AD's that have been complied with, etc.
In the end, yup, I have more money into my Mooney than I ever will get back out of it. But, just like boats and other things its the other value that you get out of it.
Now...seriously this weather needs to cooperate so I can fly it! (50+ kts winds today )