Macron is flouting French government traditions, as he seeks to impose his will on the foreign service. Considering Russia's ongoing war in Ukraine, this was a terrible time for Macron to make such a decision, and he will only decrease the quality of the foreign service. Potential recruits will not want to sink hours of hard work into a job that no longer has the perks it once did.
The French government simply wants to attract new and diverse candidates to the diplomatic service by opening new routes to the ministry. Importantly, diplomatic service members will still be considered elite public sector workers - they are only losing their special status.
There have been serious issues between the French Presidency and the diplomatic service for years. Macron has grown weary of the service, as he believes that it has worked against him - he even appointed career diplomat Catherine Colonna as foreign minister to appease the diplomatic corps, but it hasn't worked.