If you have one microphone of professional quality, then you can make recordings of a professional standard. I've heard the evidence of this so many times I couldn't count. But since microphones do all have their own individual sound characters - even identical models as they age differently - it is always good to have a selection of mics to choose from, to get the absolute best out of the sound source.
It has long been an accepted piece of recording wisdom that you should choose the right mic for the job, by experience and experiment, and find the best position in which to place it (always by experiment). To choose a mic at random and position it approximately, and then try to fix up the resulting sound with EQ just wouldn't be the right thing to do. It would be like overcooking the celebration turkey and expecting to correct the inevitable dry texture with extra gravy.
It is a fact however that EQ is often necessary, no matter how correct the microphone choice or optimum the placement. Perhaps the instrument itself needs a little work. Maybe the acoustic isn't perfect. Maybe you want a sound that is different to the natural sound. All are good reasons for using EQ.
But with some mics, it seems that the function of EQ is mainly to correct the problems that you hear. The difficulty with this is that it can put you into a 'problem-solving' mindset. And once you have solved the problems, you leave it there.
If the microphone is more neutral however and doesn't raise any problems, you can - right from the start - use EQ to improve the sound that you hear. The problem-solving mindset can be the nemesis of the creative mindset that is needed to get the best end result in the recording studio.
Opinions are subjective and differ vastly, but I would cite the Neumann U87 Ai as one example of a microphone that I find quite plain. Put it in front of a singer or instrumentalist and the sound that comes out is pretty much the same as what went in, plus a touch of the large-diaphragm sheen you would expect from such a mic.
But I find this sound very easy to work with and get exactly the results I want. With some other mics my first thoughts are the problem areas. These mics may be of professional quality and capable of professional results, but I'm thinking in a technical, problem-solving manner, and I'm not using whatever creative abilities I might possess to their maximum.
So if you have a selection of mics available to experiment with, don't just compare their sounds flat and unaltered. Experiment with the way they work with EQ too. There are so many dimensions in the technique and artistry of microphones that it is possible to learn more and more throughout one's career without ever exhausting what they can do.