The Trinity study I believe also included high expense ratios, which now are no longer as prevalent. However, there are also many, many top economists saying "be careful in being too optimistic about long term returns." Basically, markets have been at historically high valuations for several years now and so that makes it more likely that future returns will be more muted (but nobody has a crystal ball).
Personally, given current valuations I am leaning towards a 3.5% withdrawal rate. I think 4% would be fine if you had some hard skills and were not adverse to jumping in and doing some part time work if needed. For me, I have a family to support and a high income and high expenses, and it would be risky to walk away in prime earning years and then be scrapping for part time work later. I might be more comfortable with 4% if things like valuations (and government debts) were more in line with historical averages.
I think a 3% withdrawal rate is too conservative. I mean, the 4% rule worked through the Great Depression and so 3.5% is already more conservative than that! At some point, life is not risk free and you just need to live your life. If you are prudent enough to amass the amount necessary to support yourself at a 3.5% withdrawal rate, you will probably also be prudent enough not to just sit there dumbfounded as all your money slowly disappears. You will make changes.
The 4% rule is supposed to be a fixed withdrawal strategy. It is most risky if the market crashes right when you retire (the "Sequence of Returns Risk"). One way to solve that is to have a bit of extra padding in bonds and to spend down that bond padding over the first five years. Another way is to cut your spending if the market turns shortly after you retire, so that your withdrawal rate is based on your new, lower portfolio, or to pick up work until you back to the amount you retired with.
Basically, 3.5% is close to bulletproof if you are prudent (which you will be, because you are analyzing this) and 4% is more risky, but as long you make sure that you can adapt so you can sort of "reset" on the 4% rule after an early market crash (either by cutting spending or by building your assets back up) you should be fine.