Nobody has suggested getting rid of Brown, simply that if we're looking to get promoted, we should be looking for an upgrade.
Brown is a versatile player, who can play CB, LB or on the left side of a back 3, and will provide excellent cover, competition and motivation for Thorniley and any potential LB we may bring in, but he played in an extremely poor side last season, and a shocking defence, and it's easy to overhype just how good he is, simply because he was the best of a bad bunch. He has serious limitations, that we need to improve on.
The exact same applies to Long, an important player last season, but if we want to challenge for promotion this coming season, we need to be looking at upgrading in their positions, it's then up to them to improve their level, if they want to be first choice, it's called healthy competition for places, and it's what brings success.
All successful squads have genuine capable cover and competition for places, driving the first team players on and motivating them to keep performing, players like Solskjaer, Butt, Phil Neville, and John O'Shea at Man Utd for example, they were never going to be starters, but they were unsung heroes in a way, who played a major role in Man Utd's success throughout that era, always there to provide genuine cover and competition, and ready when called upon. It forced the first team to not let their levels drop.
The season is long, and there will be injuries, suspensions, and losses of form, and without these types of players in your squad, first team players will lose their drive and motivation, and will become complacent, and their levels will drop, as we've seen in recent seasons, due to poorly built squads, lacking in depth of quality and competition, and fitness.
Brown and Long are massively important, versatile squad players, but if we want to push for promotion, in the starting lineup we need better.