You've been playing too much Football Manager, it is not that easy.
Newcastle have done very well with their transfers, but it also comes down to luck and how them players will adapt to the English game, fortunately for Newcastle they have adapted superbly. Doesn't mean the next player will be half as good even if they have performed in another country.
Of course, Newcastle could just be overachieving and be rubbish next season
This is entirely the point, I don't play any football manager, some players will suck, this is true if you buy 35mil players, 50mil players who have already played well in this league, or free transfer players.
They way you stop this being a problem, is to account for it. how do you account for it? Maximise profit, if you get an offer above and beyond what a player is worth, that same player might break his leg next year and never play for you again, the ONLY way to get the full value is sell when someone wants to stupidly over pay.
If you do so you make money, you make a profit, that gives you some room for error in the transfer market.
Sell Tiote for 30mil, buy 3 players for 10mil each, if 1 is crap, you've lost 10mil. If you don't sell him, and he has a crap season then misses two seasons injured then leaves on a free, what do you get? its offsetting the risks, taking advantage of a great deal by selling way beyond his value can in most situations simply not backfire, its banking the money while its available not praying to god that value stays the same for a long time. Again this is also assuming you actually have a real say, if the player WANTS to leave then you've got nothing to lose anyway.
The more profitable you can be, the more room you have for a 10mil buy to go bad and to still be able to afford to offload that player and buy someone else for 10mil.
Clubs that run beyond there means or so ridiculously close to breaking even almost always get into trouble, clubs that remain profitable with a comfortable margin simply stay safe and well run for a long time.
It's the clubs who desperately hang on to players by giving them silly money they can't afford or buying new players without moving others on who get into trouble.
Arsenal do this time and time again and look how they are regarded financially. ADebayor on the cheap and let him go for a great fee, Toure, Nasri, Clichy, Fabregas, they've made a bunch of blunders as well and why are Arsenal getting worse. Because we started also losing players on free who could otherwise have helped or been highly profitable. Gervinho had one decent year in a poo league and we spent 12mil on a guy who (i'm assuming) was pretty damn cheap to Lille, this is the opposite of what Arsenal normally do, get a great year or two out of someone like Gervinho on the cheap and let someone else pay over the odd's for him.