Well there’s lots of things that I’m surprised you haven’t noticed so I’m sure I’ll just be repeating from earlier in the thread:
Simple market forces mean that prices for English clubs are inflated whilst prices for foreign clubs deflate once our window closes. This includes the variation in competition for players on the demand side which push prices up, and the negotiating power foreign clubs have on the selling side due to the onus being on English clubs to do or die before the end of their window. For example Barcelona were widely reported to be offering Coutinho to English clubs for a £20m loan fee, but once those clubs left the market they made do with a £7.75m fee from Bayern Munich.
Then there’s the issue of the hard time it takes to conduct complex high stakes negotiations worth many £m’s, the finite nature of transfer budgets meaning these negotiations are part of sequential transfer strategies. In this way when having to work through the negotiations for a Plan A player, then having to move on to Plan B, Plan C etc every second available is gold dust, and therefore when these seconds are diminished it either times out the deals or - as above - requires even more money to be thrown by the buying club at greasing wheels. For instance PSG had an extra three weeks to complete the complex deal for Dybala than Tottenham in case Neymar left, and by reliable accounts were negotiating at around £55m rather than the £65m fee Tottenham agreed.
This issue also applies for Premier League clubs wishing to sell and replace a particular player as foreign clubs too have to work through their transfer lists and may not come to want this Premier League player until after the PL deadline, either preventing a replacement being bought and weakening the team for at least half a season, or preventing the sale completely and hindering the squad turnover and therefore progress. This can be seen in the case of Eriksen - Tottenham essentially publicly acknowledged they would be forced to sell him were an offer to arrive due to his contract situation but would no longer be able to replace him, seriously weakening them. And if the PL window was in line with Europe's who is to say that Eriksen would not have chosen to join yourselves knowing that no rival European clubs would be coming for him due to the proximity of their window closing?
Then there’s the issue of the transfer domino effect; as I said deals take a long time and so if Barcelona for example had bought Neymar at the last moment and urgently needed to push someone like Dembele out for a cut price, would you have been happy for Manchester United to be paralysed against making a move due to the closure of our window?
