"If you're careful with a pry bar and patient enough to score thoroughly with a utility knife, you can manage without one."
Fair point, but have you considered the energy efficiency angle? The thing about older windows is they're rarely installed cleanly, and you often find hidden gaps or insulation issues once you pry them out manually. Scoring and prying can sometimes mask subtle structural problems or damage that might affect sealing later on. I've found that using an oscillating tool can give you a cleaner removal, preserving the surrounding drywall and framing integrity—key factors when you're aiming for optimal insulation and airtightness.
Also, when you mention patience—sure, it's doable without power tools, but isn't the goal also to minimize disturbance to the existing wall cavity? Every tiny gap or crack you accidentally create is another spot you'll need to seal meticulously later. Just curious, have you checked your air leakage after installing new windows manually versus using the oscillating tool? Might be worth considering if you're aiming for maximum efficiency.