Severus's so-called "strategy" was actually to score as many points as possible with the Quaffle before the Golden Snitch was caught. After all, the Golden Snitch was worth only 150 points. If they could widen the gap beyond 150 points, then even if Harry Potter caught the Golden Snitch, they still would not win.
That meant the entire team had to take part in aggressive contests for the ball. They could no longer play as they had in the past, with the whole team focused on defense, merely passing the Quaffle around without actively shooting. The Keeper had been practically ornamental, and victory or defeat had hinged on the Seeker.
Harry's Firebolt had made many people lose confidence. Not every Quidditch player had a broom on the same level as Harry Potter's obsolete Nimbus 2000. Some were even still using the Cleansweep series provided by the school, antiques that had been "passed down through generations." Ravenclaw's female Seeker was still using a Comet 260. In the words of Gryffindor Quidditch captain Oliver Wood, compared to a Firebolt, the Comet 260 was simply a joke.
If they could not win the championship, what was the point of fighting for second or third place? Slytherin had always won before, then Gryffindor had "pulled one back." Although it was not their own House that had defeated Slytherin, students from the other Houses still felt it was worth celebrating. Now it was Gryffindor's turn to keep winning, yet no one could count on Slytherin to win it back.
The new tactics meant that the importance of players in every position on the team had changed. At present, none of the Houses paid much attention to selecting Keepers, especially Gryffindor. They had placed all their hopes on Harry and his Firebolt. If they did not