I wish that it was BS. I don't think it is, as I have done some research. Everything I have found points to this. This blurb below is from the Marvin Window website. One of the better window manufacturers.
Tell me more: When you choose insert window replacement, new windows are installed within the existing window frame. Also known as “frame-in-frame replacement” or a “pocket window,” it’s an option when your existing wood or aluminum window frames are structurally sound and you want to preserve existing interior and exterior trim. The existing sash, operating hardware and covers are removed and the new window unit is inserted into the old frame where it is anchored, insulated and sealed.
Because existing trim and siding are not disturbed, this is generally a quicker installation and typically a lower cost replacement solution. An insert window is a great option when you’re happy with the size, shape and operating style of your existing window.
Even relatively new vinyl windows—less than ten years old—can suffer from seal failures and degradation from exposure to weather, and may need to be replaced. Note that insert replacement is usually not an option when replacing vinyl windows since the vinyl frames commonly lack the structure to support a quality and long-lasting replacement solution.
I have 3 (possibly 4) window contractors coming on Friday:
Pella
Renewal by Andersen
Ace Improvements (Local contractor - been around a long time - They tell me they have replaced the windows in many of the homes in my development - I think they take the windows out from the outside - remove the siding and reinstall)
And another local contractor - if they call me back with an appointment.
I am curious to see what they all say.
Joe