Whatever one makes of the drama of 2019, it's important I think to decouple the FTPA from all of that. What's more important is the answer to the question "Should the timing of the election be (mostly) up to the Prime Minister, or should it be essentially fixed?" In most countries, after all, elections do run on a fixed cycle, and if there is somehow difficulty in governing then the parties involved are expected to come together and work it out in the first instance, and only if Government totally collapses might an early election be called for. An intransigent opposition can't be an excuse -- because, after all, what is the Opposition for if not to make the Government work as hard as possible to justify and implement its policies? Even if they had to give way on Brexit in the end, the details were still important; it can hardly be sensible to claim that anything that had "BREXIT" as the title was therefore totally fine no matter what it contained underneath (and, of course, we are discovering even now that Johnson appears to regret the deal he reached...)
I don't want to go too far down the rabbit hole of exploring Brexit again, though. In any case, even if OG's analysis is right, it still doesn't mean that Johnson should have had the sole and unmitigated power to call an election. That's why the FTPA handed the power in the first instance to Parliament as a whole, by either passing an "early election" motion, or by defeating the Government in a No Confidence vote.