The concern is because we are now aiming to do that at the top level. And a bad transfer window for 80% of the PL is a potential relegation battle.
Naturally first season up without real depth and having not replicated what you suggest yet at this level people are naturally going to be a bit nervous about potential sales of top players.
It'll probably be harder to achieve what we've done before at the top level, for various reasons. Succession planning and spending more will likely have to happen more, which also means bigger gambles.