Football had already started to change by this time and teams were not staying together for as long as you made out in your earlier post. Freedom of Contract had been in place for about 6 years by this point, the effects of which we had already felt by the loss of players like Thomas, Gillespie, Hateley and Pearce in previous seasons. Even so Sillett only made one change, so he was hardly tearing the team apart!
This particular team peaked in 86/87, but nevertheless Sillett was right to try and keep the core strength of that team, while slowly seeking to pep it up with more quality. If you think that we would have done better in 87/88 by not signing Speedie, and sticking with the 86/87 side you are living in cloud cuckoo land. Their career records show that 86/87 was as good as it got for the likes of Downs, Pickering and Houchen. Yes Bennett, Phillips and Sedgley later left in 1989, but Bennett had started getting recurrent injuries and wasn't as effective as before, while Sedgley wanted to leave when Arsenal and Spurs came sniffing, so not much we could do there. I agree though that we did perhaps let Phillips go prematurely.
In terms of 87/88 we had Smith come in from within the club to replace Pickering, while Speedie initially replaced Houchen up front, until we signed Bannister later in the season, from when Speedie moved to a more midfield role, where he was much more influential and effective. We never replaced Downs which was 'the' glaring weakness in our team. The other thing that effected us badly in 87/88 were injuries, which were constant throughout. Our worst spell of the season through October/ November time coincided with the absence of Peake and Kilcline from the team. We then lost McGrath for the season, and had a stream of niggling injuries to others preventing a settled side. To finish in 10th as we had done in 86/87 was quite creditable given those circumstances.
In 88/89 we had our strongest league performance, with Speedie being far and away our most influential player, and we should really have finished 5th. Without Speedie that season we would have been mid table at best. The following season was not as bad as our final position suggests either. We got to the semi final of the league Cup, signed Gallacher, who was a quality player, and Drinkell who was a big disappointment, but on the face of it beforehand had looked a good signing. Until mid March we still had aspirations of a European place, but we had an awful end to the season, picking up only 5 points in our last 9 games.
That was the sign however, that the 86/87 team had largely had its day, and it was not long into the next season that Sillett was replaced by Butcher. That didn't turn out to be a great decision of course, but even if Sillett had stayed he would have had to dealt with changing an aging team. He might have sought to do it more slowly than Butcher, but he would still have had to start thinking about replacing the likes of Killine, Peake and Regis, and how to ease them out of the team with minimal disruption.
Given this history I just don't buy that we broke up the 87 team prematurely, or that we were wrong to buy Speedie. If anything we were too conservative and should have brought in more players.