I did not sell all but only some of them to keep my top 14 as low as possible.
I did keep all my best players but to make the average of your top 14 low you have to sell some good ones to make it happen.
Like I say, keeping a few academy players season after season means you can have a very low average once you've sold on the final day, but still have at least half a team of decent players to be competitive in the first round of the Cup, then buy in on day 3.