To estimate the market potential of your engine, you might want to look at what else there is on the market. When you are going for a general-purpose game engine, then you will be competing with engines like Unity, Cryengine or Unreal Engine which are all available in a price range which is very affordable to the target demographic you mentioned.
Don't take it personal, but I doubt that a two-person team could create anything which can compete on that market.
When you say "Well, these are 3d engines, let's do something simpler and stick to 2d" (relevant again due to the smartphone gaming boom): That market is covered as well. There are Clickteam Fusion and Game Maker for example.
However, what could work is when you create an engine designed for a very specific game genre. There is, for example, RPGMaker which is a game engine especially made for creating 2d JRPG's. It is a far less ambitious project than a general purpose engine, but still has a large userbase because the one thing it does, it does extremely well (and that for over 20 years now).
When you find a different genre without a specialized engine yet, then you might find a market niche which you can fill even with just a two-person team.