You are making a good point: selling a KM camera will get more and more difficult. Thus ifyou intend to upgrade your DSLR equipment within the next years, the D50 is certainly the better choice.
I wouldn't be so sure about that. I don't think anyone knows how these cameras will compare in the used market in the future. A lot probably depends on how future Sony products are accepted using Maxxum/Dynax mount lenses.
If they are a big hit (and Sony has already stated they want to capture 25% of the DSLR market), then they may be more valuable than you think for photographers going to the used market for backup cameras.
Just because there will no longer be any KM Branded DSLR models doesn't make them stop taking good photos either, and they'll still have the advantage of anti-shake with every lens, incuding bright primes (something you can't get with another DSLR manufacturer, at any price).
But, I'd never count on any camera body being an investment. Technology changes too fast. On the other hand, lenses hold their value much better (and used Minolta lens prices have gone up significantly since the KM/Sony announcements here in the U.S., probably due to panic buying from existing KM DSLR owners making sure they get the lenses they want). Some lenses have doubled or trippled in price on the used market, compared to what they were selling for a few months back (going up, not down).