You forgot Keita. Thiago/Keita already rotate for one spot.
True, but in Keita's best season here by a long way in terms of fitness, they started 31 league games
combined. We were actually pretty lucky their injuries dove-tailed so nicely last season - they missed 9 and 12 league games but only 3 of those occasions overlapped.
Most of our dropped points last season came when we were unable to field any combo of 3 of our 4 best midfielders - in the 10 games we dropped points, in only 2 of them would you say we were fielding a "first-choice" midfield and one of them was the hardest game of the season (City away, with the other being Spurs at home). I mean it's ridiculous that you need to be near-perfect across a season and get 90+ points to win the league, but it is what it is, and we're dealing with small margins.
Outside of the core four, the others options just aren't the same level/fit. And I'd have no concerns with that whatsoever if any of Henderson, Thiago or Keita were reliable fitness-wise. If anything, I'd think we could reduce the number of players we have in midfield - four genuinely top class players for three positions is on the face of it very good. But as it turns out, all of our best midfielders have unreliable fitness. Some seasons we'll be lucky. Some we won't. I'm not saying we need to have 6 world-class players in there, but I think we should be looking for someone in the middle ground of "top-class, but made of biscuits" and "consistently fit, but unproven at the top level".