Thanks for your observations! Will probably give PES 2015 a shot with a good patch eventually! I'm a PA1 player so it'll be an interesting experiment to see how passing works in this game.. I'll get to finding a good patch eventually, as I think that's half of the experience right there, but from what I've read in your comments, it definitely seems like worth checking out.
I was thinking the same questions the other day, as i played some ML in both games, where i have a fresh save.
I have 260+ hours in PES 2015 PC and about 960+ hours in 2016, while i am sure most of the 50% is editing time in PES 2016.
Well 2015 is a pretty solid game. It was a lot back to basics, when it came out. PA1 is very good, works as it should , the physicallity was again in PES standards, after the 2011-2013 period , which went in a lot experiments.
PES 2016 is a more simplified 2015, i am experiencing this again now, after i came back, 6 years latter, i have a tough time in 2015 where i am missing the learning curve, but 2016 is like people's PES. It is still like, i was playing it every day.
Also PES 2016 is more responsive, to a 2018 level, with less smooth animations and more physical game.
The defending AI and GK AI is like
@rockstrongo said, almost non-existent.
I am pretty sure, 2016 suffered from the marketing campaign of that year. After the very late release of 2015, they wanted to rush the production, and throw every marketing gimmick they had, that's why the 20 years anniversary label is here.
This resulted to shortcomings in gameplay, and in content, if you remember, the rosters were Live Updated, almost a month after release, and some rest of Europe teams, were added with a future Datapack. Which led to a lot backlash and criticism back then. Practically, the game launched in September and you couldn't start a proper ML, until late November.
Also a personal hint here. I am pretty sure that the 2016 was so rushed, by the stupid deadlines KONaMI pushes and a very good FIFA that existed that years 2014-2016, which additionally pushed the PES productions team for sure. I believe that the 2016 is the most forgettable game for the PES productions team, i believe nobody remembers anything by its development.
Finally in content, here's where i will disagree with
@rockstrongo . PES 2016 was the most full of official content, until PES 2019 arrived.
It has the 2015 package, Serie A, Juventus, Man United, licensed, La Liga licensed, Bayern Munich with Allianz Arena ( yes it wasn't new feature in 2020, it was there since 2015

) , Bradileirao, Liga Argentina, and the difference with 2015, it has again Liga Chilena, which Liga was in 2014, removed in 2015 and back again in game in 2016.
Then a full licensed Libertadores and Sudamericsna, the equivalent super cup of Latin America, Recopa Sudamericana. I miss so much those Latin America domestic competitions, i think they add a lot to the game, and should exist in latter PES games, even as fake, not only the Fake Libertadores.
And of course the AFC CL. Add to this, that those competitions, Libertadores, Sudamericana, AFC CL, start in February/March, so KoNAMi took full advantage of the license that years, 2015-2016, having for example in 2016, both the 2014 and the 2015 versions of that Competitions, both as a standalone game mode, and as additional teams in Master League.
Due to this , you may see in edit mode that some teams may exist 3 times, like i think Deportivo Cali or Atletico Nacional, but in ML , there is a duplicate buster, so they appear only as one.
And finally 2016 has the Euro 2016 datapack, the last one, which according to that years lore, came finally FREE DLC, as an apology for that years inconvenience by KONAMI.
But it comes again in a KONaMI way, not all European nations inside, not all present EU teams licensed and from official EURO stadiums, only Stade de France.
In short terms, content wise, PES 2016 has what PES 2015 had, plus Liga Chilena and EURO 2016.