Take the share out of the equation - If I have a C150 and buy it for £16,000 fly it for 1,000 hours and put some cash to one side to cover a new engine, do I sell it for £16,000 and give them the engine fund? no i sell it for a figure commesurate with what it's now worth (say £14,000). It may equate to what I have in my engine fund, it may not.
You are right in the drift, but you won't get 14k for a C150 with a genuinely run-out engine. And if your engine has done 1000hrs and your fund is 16k, that implies a C150 engine can be overhauled for 32k. Actually it can be done for far less than that, and your fund is well overfunded.
Look at the prices paid for old piston twins - close to the engine value; the hull is close to worthless.