Exactly! The stock has been a terrible investment since IPO, and Twitter management has done a terrible job of unlocking value. It's current management that has failed to keep up with the broader economy. Musk thinks he can do a lot better if he has full control. Musk's price is a reflection of how poorly current management has done; he is not trying to compensate you for future growth.
If you think current management can drive the share price much higher, then you should not sell to Musk. If you think current management is just going to be more of the same, Musk is giving you a big premium over where the stock had been trading, and you should take his offer.
If Musk thinks Twitter will be worth $400 in five years' time, then he can realize that value for himself. He'd be very foolish to offer you that price now.