How did Monaco do well, they sold all theirs stars for ridiculous amounts of money but that wouldn't impress the fans. They also won the league by about 15 Pts, so why would they have not competed against PSG? Even with PSGs signings they would have still put up a good fight if they never sold anyone.
If Arsenal won the league then sold all theirs best players the following year you would be ripping Wenger apart. Bit inconsistent with your views.
Mbappe isn't a guy that has played first team for Monaco for 5 years and they sold him on a whim, they got 180million quid for a guy who might go crazy with bigger money or might basically do anything, or could be brilliant but would leave eventually anyway.
They didn't sell players for cheap, they had an owner who wanted to massively bring costs down a few years back and has engaged in a process of identifying younger players to bring through, this is part of the process, financing the club going forwards and Monaco couldn't be doing better at that. They sold players, brought youngsters through, those youngsters won them the league then finance the team for the next 5 years while they bring more youths forward.
I didn't say they couldn't compete with PSG, but PSG had a bad year, they were exactly 8 points behind and previously beat Monaco by 30 points the year before. PSG have strengthened immeasurably when last year they lost Ibra who was an incredibly important part of the team, he made things tick offensively. When Bayern have an off year and Dortmund win, you still know Bayern will win the league 9 out of 10 times the next year, and the next 10 years. That is just how it is, Monaco are a monumentally smaller club than PSG, they'll have good and bad years.
If you ever actually paid attention my stance has always been, for Arsenal, for Liverpool, for Monaco and for Real, clubs sell players, only fans and bad managers are scared of selling players, it's business, players have a price, if you can get a ridiculously inflated fee.... sell, it's better business. There is ALWAYS a good new player. People said Monaco were making a mistake selling Martial, but look who came after. There are always more new players and great players. If you have good scouting and great coaching you can always produce more top players. It's only when your scouting goes to pieces and your coaching goes bad(Arsenal, on both counts) that hanging on to players becomes important.
Monaco can't win the league every year against a state sponsored club, simply not going to happen, what they can do is peak, get insane value for players when clubs are spending stupid then scout, coach and make new great players and have a pop every few years.