If there are those that were strong enough to run BiTW solo and gain money from it with alts, could they not do the same with any other shadow mission, party quest, part time job or even field boss guild quest?
Yes and no. I am trying not to defend hard working grinders out there but there are not many SMs where you predominately spam Windmill to gain high yields of golds and exps at the same time.
On top of that, clearing BiTW int level was done under 5 mins for many grinders ( I am saying this but many do finish around in 3mins 30sec or less), it will take considerable amount of time for hard working grinders to adjust to a change in exp/gold income. Eventually, people will find adequate dung/SM for their grind session, but not massively yielding as BiTW and possibly have to change the grind style from repeated use of Windmill to a bit more dynamical plays.
My stance on BiTW is that the nerf was unavoidable and it came tiny bit late. It served as a good foundation for new comers to build a baseline of gold/exp income when it first released. Did it's job when server's peak population was 50-60 on Saturday and you couldn't find a consistent party for a grind at your power level. Now that average server population has changed and people can actually one shot mobs for a mass grind and so on, it did last long and potentially create harm on the newer player base who have recently joined.
I strongly urge you to reconsider the direction you've taken with the changes, which, in my opinion, have only crippled party-play with other people in the long run.
It really depends on who you are and how much you are opt-into solo or party play, some may balance it out. You cannot force every solo grinders into a party play everyday and vice versa.
I am sure that the history will repeat and there will be somewhat exploitable content to pop up because unbalance is the outcome of balancing in mmorpg. As for now, let's enjoy the new cor content because it nets good for sure.