The data is out there, you just have to dig (more so than I had to back when I started med school). Your numbers are a bit off, while you're right, US MD graduates stayed relatively stable from 1985-2005, in 2006 to the present US MD expansion had initially increased slowly, but accelerated in order for them to meet their goal of increasing US MD seats by 30% by 2016 (modest goal of 4500, which they barely missed by 500 seats, but have since surpassed). In 1985 US MD grads were`~15,000 and in 2010 they increased to 16835 (increase of ~1800), and this trend has continued with 19553 graduates in 2018 (but over 21k matriculants). Based on this, total expansion of US MD seats from 1985 to 2018 was in the realm of 4500-4600 (can't get the exact number for 1985).
As for DOs, there were 1476 graduates in 1985, and this certainly did increase, but did so only modestly initially with an increase to 2708 by 2005 (meaning while US MD schools remained stagnant, DO schools increased by ~1230 seats), but again comparing 1985 to 2010 DO grads went from 1476 to 3631 (increase of 2155, compared to a US MD increase of ~1800 in that time). If we look at 1985 to 2018, the total number of DO graduates increased from 1476 to 6416 (increase of 4940 seats). So actually, we are talking about a discrepancy of only 300-400 seats over the last 33 yrs.
For the numbers on these I used these below:
-NRMP report from 1985:
https://mk0nrmpcikgb8jxyd19h.kinstacdn.com/wp-content/uploads/2013/08/resultsanddata1985.pdf
-AAMC total graduates report:
https://www.aamc.org/download/321530/data/factstableb2-1.pdf &
https://www.aamc.org/download/321532/data/factstableb2-2.pdf
-AACOM total graduates report:
https://www.aacom.org/docs/default-source/data-and-trends/1985-2018-gradre.xlsx?sfvrsn=45724597_106
Also regarding attrition rate, historically it has been ~8%, but some have argued its closer to 10%. They haven't released an attrition report in many years, so its hard to know for sure, especially with adding seats every year, but someone with more time could look into it and get a crude number based on matriculant and graduate numbers.
500 seats is certainly a lot, and I believe if that is the new trend (essentially adding 3 new schools every year, which first happened in 2013 - grads in 2017 - but seems to be happening more and more) that is a problem.
But its also difficult to look at any individual year. Its a gradual process. I have no idea if there are US MD schools in the pipeline, but it certainly seems like we are on track to further exceed their expansion. Again though, if you look at the time that US MDs were actively expanding (2005 to the present) we're pretty much neck and neck still in terms of absolute numbers (~4000 and matriculants ~6000). It may change in the next 4-5 yrs, but as of right now, those are the numbers.