Wouldn’t it be best to make the mix as good as you can, set it aside, tweak, and then master???
Why master and then tweak? You’ll just have to have it mastered again. $$$
I agree that the mixing level has improved greatly.
I encourage everyone to keep mixing and mastering, if only to improve your skills. Fantastic.
Well as you make it clearer and clearer to hear the detail (break out the clearer lens to see through analogue), the better it will be all around. If you are looking to save money and don't want the best possible way to launch your project and musical creation, you can skip Mastering, mix it yourself, play all instruments yourself etc.
To repeat what I said earlier IMHO "Making certain that you are rested and coming in with a set of neutral ears is great preparation but not a replacement for a set of Mastering skills."
In addition to Mastering skills, "real" Mastering setups have a much better room, setup and playback system than what was used to Mix the songs.
It is a huge investment and step that most would rather save their money on, at least until they believe their skills are getting to that point.
Have a look at this Master studio layout, speakers etc. and his comments around 22:30 "normally mixes are version 5 or 6 when they get them...." The complete ideo is very interesting and if oI couldn't follow along the conversation and understand exactly what they are saying, then I'm not ready to Master at that level.
But that's just MHO and experience.