Seriously? I don't think Barzal is going to regress, he's a freaking sick player, holy smokes, but your own examples clearly show the difference in competition. These graphs are absolutely terrible because they have no numbers and are there for visual interpretation. But even when you look at them, Tavares faced well above average time against #1 and #2 defensemen, average 2nd pairing time, and well below average third pairing time. Barzal faced below average time from #1's, above average time from the second pair, and average time from the third pair. The forwards are similar, Tavares was well above average against the top4 opposing forwards, while Barzal was average for #1's and below average for 2-4. Tavares was well below average in time against 8-12, while Barzal was above average against the #9th and 10th forwards. There are other little variances throughout, but those are the ones that jump out at me. When you add all those up, it is a huge difference in competition and to claim otherwise just means that you aren't interpreting the graphs correctly.
TOIQoC
Tavares - 29.77 (1st for Islanders forwards)
Barzal - 28.94 (5th for Islanders forwards)
Tavares was 12th in the league for forwards with 50+ games. Barzal sits closer to 200 with neighbours like Marner, Konecny, and Tyler Johnson.
Again, I don't think Barzal will regress, I think he's phenomenal, but bad analysis does no favours.