Going back a couple of days here, but...
To be fair, you originally stated that "we've consistently brought in players like Alli" and when pressed for examples for named Eriksen and Lamela. Neither were anything like him. Lamela had just come off his second season as a starter for Roma, a season in which he scored 15 goals playing primarily as a right winger (and cost you £30m). Eriksen was just about to embark on his fourth season as a starter for Ajax, but only had a year left on his contract, and was keen on moving on, so you got him fairly cheaply (£11m for him was an amazing bargain, truth be told).
Point is, both were known quantities, as both were highly rated young players that most people who followed football with any kind of regularity knew of. They were already starters for fairly good teams, and I'm going to be so bold as to say that both were probably expected to be able to contribute pretty much right away (hence Lamela being labeled a flop after failing completely to do anything of note in his first season). Alli was the basically the complete opposite of that. He was a virtual unknown (though he had started to attract a little attention on account of the great season he was having), playing in League One for MK Dons, and cost you just £5m. Luckily, he adapted to the league pretty quickly. Had he not, though, it wouldn't have been that big of a deal. He was a 19 year making a big step up.