This is an argument between R Meir of the Mishna and R Shimon Ben Yochai
But the Rambam seems to make an amalgamation of the two opinions.
The argument is in Sanhedrin 63.
R Meir said, "If not for the letter vav in 'These are your gods,'
(which was said to the Golden Calf) Israel would have been liable to be destroyed."
[The vav makes it plural. Without it it would have been "This is your god" referring to teh Golden Calf. With the vav they were joining G-d with the Golden Calf]
R. Shimon said, "But anyone who joins the name of God with something else is uprooted from this world as it says 'to God alone.' Rather the vav is to tell us they desired many gods." [In Avoda Zara it is explained that that means they accepted the Golden Calf but were open to accepting other god also. But they did not join God with the Golden Calf. And if they had that would have been worse. ]
The Maharsha says that joining is what the Rambam describes at the beginning of the Laws of Idolatry. And there the Rambam says the main idea of idolatry was they saw that God put the stars in Heaven so it is his will that we should honor them just like he honors them, and by that they will be advocates for us. [The Rambam goes into detail about this also in his commentary on the Mishna. This is known in the as the problem of the אמצעי intermediate. That is people know God is the creator but they feel they can't approach Him directly so they go through a middle step like a person or anything else to serve as a middle step.]
Then the Rambam says the actual idolatry that we know came after that. It seems the rambam is saying the later step was worse. That is the אמצעי (emtzai) (using an intermediate) is less serious.
But then when you look at the Rambam about actual שיתוף joining --in the only place he actual brings up our Gemara-he says one who swears by God and something else will be uprooted--that is the opinion of R Shimon. Not like R Meir!
So what we have here is what seems like a contradiction in the Rambam.
I should mention that there are people that do not consider going through an intermediate as a problem. There are groups that think this is OK. But I think the Torah is right that this is a problem.
I don't know why people ignore the Torah in this detail, but to me it seems like a serious matter.
I don't mean to be critical of any tzadik. But even a tzadik should not be an intermediate.