Development is not going to change from a pure monarch point cost, period. It'd have far too much bleed-over with buildings and would defeat the entire point of letting small countries develop when they can't expand.
The problem is that development rewards countries for doing nothing. Countries that sit around doing nothing and get lucky enough to have a god-king can enjoy growing stronger with no risk or agency. There's no tactical thought or skill, merely sitting on speed 5 and mashing development every couple months. Anyone who's tried to play "tall" knows how boring it is and nobody could think it's an adequate replacement for expansion.
Monarch Points remain a currency that can barely be influenced while ducats are a much more influenceable currency that is considered completely inferior and worthless when compared to MP. Ducats only serve limited use in maintaining an army/navy (which are often limited by manpower and merc pool), constructing buildings (which are often useless without development) hiring advisors (to generate the infinitely better MP) and creating a buffer for certain negative events. Once the player hits the critical mass of ducats needed to maintain those services they no longer serve much point. There's essentially little to motivate the player on maintaining a strong economy aside from protecting your MP pool.
Linking development to ducats creates positive feedback loops that rewards players that maintain their economy, makes ducats a significantly more useful currency and makes sure that countries need to expand their sphere of influence to maintain their relevancy. This can be easily coupled with a centralisation mechanic that offers bonuses (and certain maluses) on reforms which difficulties scale on nation size. Small countries will be more easily able to centralise their countries (which offers easier development) if they're willing to commit which will help them compete with majors who will stagnant if they choose to ignore investing resources into centralising their realm.
This still allows countries to "play tall" but actually forces player agency and more importantly adds depth to peacetime and simple domestic policies which the fan base has been begging for and Common Sense failed to deliver on.
This also allows development to be much more dynamic with factors/events that lower or raise development. Right now with MP being such a sought after resource losing development in a province would come as a harsh and unfair blow. Instead having it dynamic while linked to a much more flexible currency which the player has a much greater influence on isn't nearly as painful and would introduce some depth to a game often criticised for being shallow.
The other issue with MP development is locking low technological groups out of what should be a core gameplay mechanic (although apparently it isn't because it's locked behind a paywall). Western to Ottoman tech tend to be completely drowning in MP with nothing to spend it on apart from development while ROTW nations either forgo technological advances (like dip tech) to dump into provinces or ignore development completely.
A lot of people believe that the game is becoming over reliant on MP and while I share that thought, I think MP makes perfect sense for the more abstract elements of EU4 like technology since it's something that was rather opaque in the early modern era with a number of contributing factors that would extremely hard to simulate in EU4. However being able to will a city out of nowhere which then for some reason stagnates your cultural and technological growth is just not a good mechanic in either a gameplay or historical sense.
tldr: just let us mod it. srsly.
