I disagree with Garland, in a vacuum, being the wrong pick. Frankly, we have no idea. Garland has not shown substantially less than any of the other rumored players at that pick.
The problem with the Garland pick is that next year is a guard-heavy class. It seems highly plausible that the Cavs will have a high pick, the best player available is a guard, and then the Cavs are in a world of bad options. They either have to make a trade (moving Garland, Sexton, or the pick), take a third PG/combo guard in the top-ten in as many years, or reach for a player that otherwise would not be taken as high.
I do believe that a title team needs a star facilitator, which is perhaps where we disagree, but insofar as I believe that then I think the Cavs can justify the Garland choice.
But, when they made the Garland pick, they should have had a more clear plan of how to handle Sexton. It still seems like the team does not know what they want to do with the Sexland combo. That's a problem, but if it can be resolved by the draft, then the team can recover.
What is a fact is that there is no way that a title team can have Garland, Sexton, KPJ, and another top-10 guard choice in their plans for a title team. Either one or more will have to be traded, or the team will need to build a team around two of those three and not draft a guard.