There is a nugget of truth behind this Window obsession. Smaller-revenue teams have a tougher time signing premium free agents, or retaining their own top players past their initial six years of team control. That puts extra pressure on these poorer teams to bring up a bunch of great prospects all at once, then hope they get good at the same time before they get expensive.
But far more often it's a bullshit excuse. It's a vague, faraway goal that always seems several years out of reach. It's a cover for cheap, greedy ownership, lousy scouting, drafting, and player development, and myopic trades. It's a weak attempt to placate a fan base screwed over by years of management incompetence and indifference.