Your problem is not "under investment", it's your scouting and player recruitment system. If the latter is not fixed, the you may well continue to spend even more hundreds of millions to no great effect.
I'm not sure what players we've recruited since Van Gaal arrived that were particularly bad value in today's market.
Martial is worth the fee we paid. Herrera is worth the fee we paid. Blind is worth the fee we paid. Schweinsteiger is worth the fee we paid. Schneiderlin is worth the fee we paid. Rojo is worth the fee we paid. Darmian is worth the fee we paid. Shaw is worth the fee we paid. Romero/Valdes were free transfers. That leaves Di Maria and Falcao who we lost maybe £20m on, which is an insignificant amount of money and Depay who in my view is a £25m flop, but again he still has time on his side and as Spurs has found out since Bale left - every transfer has a chance to fail. There is nothing wrong with our purchasing or scouting departments since Van Gaal arrived.
The problem we had is that we needed to build a completely new squad of players from almost scratch. The only top quality players left from the Fergie era in our squad are Smalling and De Gea, with Valencia/Jones/Rooney/Carrick/Young arguably adequate backup players. That means we needed 9 first team quality players and probably 4-5 squad options between 2013 - 2015. That's at least 13 new players required in three Summer transfer windows, one window of which was completely wasted by Moyes.
So yes, our squad did have years of under-investment. We're competing with Man City who had a better squad to start with and have spent £260m net since Fergie retired. Fergie spent a fraction of this in his last 4-5 years. Since then we've started to redress the balance but obviously this was always going to be costly. Replacing nine first team players at £30m each would be £270m, along with 4-5 squad options you're looking at very little change from £350m, Of which Van Gaal has spent around £240m and another 2-3 players at £110m should have our squad where we need to be.
As an example lets look at City's squad as a benchmark of the cost of a title winning squad: De Bruyne £56m, Sterling £49m, Otamendi £32m, Mangala £44m, Bony £28m, Fernando £12m, Delph £8m, Navas £23m, Fernandinho £30m, Clichy £7m, Aguero £38m, Nasri £22m, Toure £24m, Kolarov £17m, Silva £24m, Kompany £7m, Zabaleta £7m, De Michellis £4m.
A total spend of around £430m.
Lets compare this to the squad Fergie left, omitting players within 3 years of retirement or blatantly not good enough (and I'll stretch this to include a declining Rooney, a poor Kagawa, an aging RVP and a "never going to make it" Zaha). Jones £16m, Smalling £10m, Rooney £27m, Nani £18m, Rafael £2.5m, Valencia £16m, Hernandez £6m, Young £17m, De Gea £19m, RVP £24m, Kagawa £12m, Zaha £12m.
A total cost of around £180m.
In the modern game you can't compete (certainly not in the medium-long term) with a team where they have a squad worth two and a half time times the amount. Unless you have a chief scout with a crystal ball who can predict the best players in the world in 10 years time, or you're prepared to gamble on youngsters and pray that as high a % as possible become top class, whilst they are causing you to drop points week in, week out because they're inexperienced and raw; then you're never going to assemble a great team for less than a few hundred million.
Naturally our other problem is we've employed two managers who've both proven to be poor selections. That's what we really need to get right third time around.