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Psyche, as far as I know, there is no open-source code. Otherwise, any programmer could analyze the code and make changes. The main problem is that only partial data is extracted from the game (from the executable file, etc.). And in some cases, one has to simply guess how and what is implemented. That is why, after making changes, a new executable file is created based on the old one. Yes, firstly, it takes a lot of time to figure out the method itself and the extracted resources, and secondly, it is inconvenient and has many limitations. If Online Hotseat (OH) was implemented in Heroes, then, of course, it can be implemented in Sword of Chaos (SoC) as well – it's just that few people have experience in delving into the resources of Heroes.
As for the "pulling the blanket over" – in the CIS, perhaps, yes, over time. In the world – I highly doubt it. Although, if there is good advertising, investment in tournaments, and coverage, everything is possible. Besides, Online Hotseat has existed in Heroes for a long time, and the popularity of Sword of Chaos is not decreasing (although many said that it would draw most players to Heroes). We have already seen different versions of Heroes based on Sword of Chaos appear, and we have also seen these versions completely forgotten or their popularity significantly decline. As a rule, such versions exist only while development is ongoing. Development ends, and popularity declines. Another question is how the developers will ensure the possibility of supporting the game by other people in the future. But in essence, these are just different games (there are more and more differences). History will put everything in its place, and we will wait for now