You forgot that United just started investing in 2014 which City started in 2008. During SAF era 2008, we didn't invest much as we do in 2014. It took Real Madrid 5 years to become a one of strongest in Europe after spending 230m in 2009 and continue spending.
During SAF era post-2008 to 2013, City spent 460m net, Real Madrid spent 312m net, Chelsea spent 260m net and United only spent 86m net which is a huge difference in spending compared to a top club. It cost us a mistake of not competing with them in spending and at that time we had an ageing squad and it needs a huge overhaul. That's why we fell quickly.
However, How many years it took City becoming stronger and how much does it take City to become the best team? In order to do that, you need to spend much in many years so you can become the best team, it worked for Barca, PSG, Chelsea, Real Madrid and City. Spending a lot of money in one transfer window doesn't strength the club to be overwhelming stronger overnight, it takes a long time in order to be the best team and continue spending level.
I could argue Moye and LVG had disaster transfer window, many of the players did not live to its tag price. Jose had brilliant transfer window barring Mihki and still need to get more quality players to make United a finished product, it's only a matter of time before we become the top club in Europe.