They sold Chamberlain when they were in League 1 and Walcott after he had played 20 games in the Championship. Bale did not look the player he ended up being until Redknapp, out of ideas, played him on the wing against Fulham. Lallana was there all the way through the leagues and now they've got far more than he'll cost to replace when he's 26 years old.
Southampton have been very shrewd. They have sold two fullbacks who between them have only played 70 odd league games for the best part of fifty million quid. The worry is that Lovren and Schneiderlin etc. also want to go. You say the top 4 aren't untouchable, but Southampton were as close to Norwich who went down as they were to Arsenal who came 4th.
The reality is that since Euro 96, only 9 different teams have made it to the top 4 - Man City, Man United, Chelsea, Liverpool, Arsenal, Newcastle, Leeds, Everton and Spurs. Of those, Everton did it once and didn't get out of the Champions league qualifiers and the less said about Leeds the better. Aston Villa, probably the biggest club in the country after those 9, splurged millions under O'Neill, and still couldn't do it. City have spent the best part of 1 billion pounds to make it there.
Swansea and Southampton are both backed to the hilt financially. Southampton have found a way of breaking even - incredibly people seem to think that this is because they are some sort of super club - the reality is, Southampton is 80+ miles from every major club with any sort of youth set up so their catchment area is massive - they've brought through an average of one quality player every 2 years, which isn't really that incredible. Compare that to, say, Bolton or Wigan who both have 4 of the previously mentioned clubs within 25 miles, and you get the picture why Sotuthampton can operate the way they do.
People will always point to Everton as a shining light, but the truth is they have the fanbase and history other clubs can only dream of. They are one of the traditional 'big 6' along with Liverpool, Arsenal, Spurs, Chelsea and Man United - the last time the top 2 of the league didn't feature one of those clubs was in 1981, the last season with 2 points for a win, when the league was much tighter. It has only happened 4 times since the 1950s, and on every occasion one of them came third.
The last time none of them came in the top 4? 1936
When Sunderland won the league from Derby, Huddersfield, Stoke and Brentford. Arsenal were the highest finishers of the big 6, in 6th place.
In fact, if you look at the top 2 from the since 1985, they have come from the 9 top 4 teams I discussed, Aston '10th best club' Villa and the Man City of the 90s in Blackburn. The last true 'outsider' to make it was Southampton in 1984.
