PSG does not need to spend much to compete in the French league. But this team's only target was never the league, they always targeted UCL and they spent accordingly. Now they have removed Mbappe and sold him, they need to make transfers to fill his place, for this they need to make a lot of meetings, convince the players and managers, work extra hours and at the same time increase the offers for the players they want.
Yes, PSG is still quite strong in League 1 even though they have lost Mbappe they also have good quality players so they don't have a problem competing in League 1, but what PSG is worried about is that when they have to compete in the Champions League, they haven't found the right players they have performance like Mbappe, they have to find him quickly before the season starts or they will fail again in the Champions League. Victor Oshimen is a player who has been strongly linked with PSG to replace Mbappe but it seems this player is not really interested in playing at PSG even though the club has made a high salary offer. Oshimen doesn't want his future to be wasted like Mbappe. He definitely also has dreams of winning the UCL title or other prestigious titles, being at PSG doesn't guarantee that apart from the domestic title.