Because you buy someone for 15-20mil, to a top four club, they are basically automatically on 60k+ a week, maybe a lot more depending on who they are.
30 players all on 80k a week, which is 4 million a year, 120mil in wages and the club stops making any money at all.
You can't buy a 15million player in every position unless your wages are paid out of a billionaires pockets, just accept that. Scholes and Giggs, VDS will be on HUGE money, like between them the saving when they retire will probably be at least 15mil a season each, the kind of wages that over a 5 year period will pay for 4 7-10million younger potential future stars and their wages. You can't just get those players, a right back, a central midfielder, a winger all today as their wages will be added on top of the rest.
You got a left and right back that for the last couple years loads of Man U fans have insisted look great for the future(I haven't, i think Rafael at the least, maybe Fabio aswell are horrible defensively).
You can't just ignore wages, wages cost many times more than transfers every year, you have X amount of players on your book, they can't all be on star wages, you can't keep adding stars, you add them as you leave. I'd expect Fergie to be looking to buy experience and quality, at a price, for Scholes, VDS, Giggs when they leave and to be looking for cheap potential stars as he has been till that happens.
Its WAY to early to say Smalling is worse than Evans, personally I think Evans gets made a fool of constantly, Smalling just hasn't had games recently, is younger(AFAIK) and played WAY WAY less games in the league than Evans.
Theres really 3 options, buy people for 20-40mil and pay their wages out of pocket, not out of income, only 2-3 clubs in the world can do that.
You buy lots of 3-10million players when they are youngish(18-22) and hope some come good, on lowish wages and sellable for 3-5mil even if they aren't very good. Call that the Man Utd plan at the moment
Buy dozens of really young players (13-18) and pay them 1-10k a week over a long period of time and hope some come good, call that the Arsenal plan.
So its something the club can't do, or two options that really do end up costing the same.
The Utd method draws from a much more defined pool of players where 80% of the potential players have already crashed and burned and aren't playing football at all anymore, or down in a really low league.
The UTD method, maybe 1 in every 4-5 players will be good enough for a top 4 team. The Arsenal method is cheaper, but because you get them so much younger WAY before you can know if they'll come good, you're talking about one in every 30-40 players who might be good.
5x 8million fee, 5x 10k a week for 3 years, or 30x 1million fee, 30x 1-2k a week for 7-8 years before you know if they are good enough. Theres not a huge amount of difference.
End of the day EVERY club will buy LOTS of utter failures, thats football, always has been. How many crap players did Mourinho buy at Chelsea who they shipped out almost straight away, City are doing the same thing, Real and Barca do the same thing, so even those guys are buying 3x 30million players, 3x100k a week, and only ending up with one "keeper" .
Smalling could be great, the twins MIGHT get good enough, they might be cheapish long term backup, you don't have a central midfielder or another great winger, but when Giggs/Scholes retire, you'll be buying some, same for VDS. Which will mostly leave you weak at right back.