There's the age old debate about Short and the amount of money spent but it's more complex than that imo.
We spent a shitload of money BUT there was a clear trend from 2012 onwards - the first summer without Quinn as chairman:
12/13 - Went into the summer needing a striker as Bendtner's loan ended. Sold Turner and started the season with only Saha and Cuellar on free transfers as our only signings then signed Fletcher and Johnson after the start of season. Both had been frozen out and arrived unfit without a pre-season - Fletcher hit the ground running but Johnson took until the second half of the season to start to deliver and said how it took him ages to adjust from being a perennial sub at City to playing 90 minutes without any pre-season
13/14 - The De Fanti Udinese model summer. Bit of an outlier as we did most of our business early but sold Mignolet and Sess for a combined £16m then signed 14 players for less than £20m combined
14/15 - Colback and Bardsley left on frees and Alonso, Ki and Borini returned to their clubs after their loans. Signed Jones, Pantilimon and Gomez on frees early on, started the season without a right back and Roberge starting in defence then spent big on Rodwell in August.
15/16 - Signed Lens and Kaboul early on for £9m and £3m but also bought Adam Matthews for £2m only to realise he was shite, sack him off and replace him (Yedlin) in the same window. Then got off to a bad start and bought Borini immediately after Short started to get stick in the ground vs Norwich, despite Advocaat asking for a targetman and being told by Congerton he could play there.
16/17 - Allardyce leaving undoubtedly disrupted things, but even at the point of the Iceland game we were one of only two clubs (Stoke the other) not to have made a signing and Allardyce has said he was still trying to agree transfer budgets with Short when he left in late July. We then spent £8m on Djilibodji in August on the eve of the season and £13.5m on Ndong on deadline day.
So imo there's a clear pattern where we ended each season scraping survival and relying on loan players who returned to their clubs, going into the summer needing significant investment just to catch up to the level we ended the season at but trying to get by with frees or cheap signings until we inevitably got off to a poor start then splurging on unfit players after the season started. The headline figures show we spent a lot but a lot of that was throwing good money after bad to correct our own mistakes after his hand was forced. Perhaps if we'd started the season with an actual fit players through the door early in the window we might have won a game before October at some point? We could have gotten away with doing that too if we weren't starting from such weak positions every year with key players from the previous seasons leaving.