I am really beginning to question the intelligence of gaming companies. There is a real opportunity to open old games' development platforms and have open contests for people to make hacks like this. Take the best ones, put them into a collection, and let the creators share in the profits. Everyone wins, especially fans of the games.
To be fair to the gaming companies, I can understand why they don't want to open their platforms. The infamous Video Game Crash of 1983 has been blamed, in part, on the fact that console-makers such as Atari, desperate to get a large portfolio of games in the market, held very low standards for what games they would allow to be made for their system, leading to outright terrible games like E. T. and worse, outright offensive titles like Custer's Revenge. The desire not to repeat this mistake is much of why Nintendo held tight control over its franchise and, in the US, deleted the edgy, religious and sexual elements from games like Final Fantasy 4/6.
Although, I'm inclined to agree with you. If Magic: the Gathering had gone open source at some point, there would still be people over 15 who play it. However, they'd also have to change their business model.