As far as finances, the depreciation of the digital will probably far surpass the cost of tuning. Technology tends to make things worth far less in just a few years time.
Speaking about depreciation, how much do you think evaporates just by the instrument becoming second-hand in a decent acoustic piano? Buy a decent Petrof, Schimmel, or Yamaha upright piano and the value of a mid-range digital piano would probably be similar to, or less, than the money you lose after trading an acoustic instrument back in after a year.
Selling it yourself would get you a better price but may take months, during which you're still stuck with it, and the instrument further depreciates and also needs servicing.
Besides, I just browsed a national auction website from my country, second hand Clavinovas for example are rarely offered, mostly only old models, and they still fetch decent prices. When I bought a Petrof Model 131 12 years ago, I paid about 4 times what I got back for it 11 years later, from the same dealer. Which was also the dealer I always had it serviced with.... and my employer at the time I traded it in.
Decent acoustic instruments are widely available, and if well-serviced last a lifetime without a complete refit. Often they only need tuning and perhaps a bit of mechanical adjustment or re-felting the hammers. The touch gets lighter over the years due to wear but we used to sell a lot of very playable and very beautiful sounding instruments that are 50-60 years old.
That's not a very good business model for a product to retain value. A piano is a consumer good that is as widely traded second hand as cars are, but one with the durability of a house.
Imagine cars being so good they'd last 80 years (and car technology being only marginally improved, if at all, during the same time) there would be one BEAR market for used cars I tell ye what
