The game engine being used does not automatically add to the performance overhead and its a design decision on the part of the games developer to cater to the needs of a given segment of the market place.
Making a game that will run on a low-mid spec PC using UE4 should be much easier to do than doing so using UE2/UE4 as to tools to do so are there. The mobile tools could just as easily be used for PC games by avoiding the over use of eye candy or technology driven API’s. So having these engines made available is not really necessary as to this need but rather the interest on the part of the games designer to make games that have a low performance requirement.
On the other hand if by low-mid spec PC’s the definition is “old” then the issue of obsolesce by design becomes a factor where the tools needed to to maintain any kind of meaningful development are simply not there or are quickly disappearing being replaced by more advanced technology. Once again in the hands of the games developer is the desire to work with older technology to produce games for a limited market and doing so they become responsible for both the game and “engine” requirements.
As for going open source I’m a fan of the ideal’s but in my experience a game engine is the one thing that should not be open sourced as to the logic as it’s use is not really compatible as to the needs of those who want to play your game.
UE2/UE3 with out Epic’s support is not a very desirable platform to develop future games on.