You already can replace the default assistant app with any app that declares itself as an assistant on Android, and have always been able to.

Google is a designated gatekeeper carrying all the same DMA obligations for:

Google Search, YouTube, Chrome, Shopping, Maps, Ads, the Play Store, and Android.

https://digital-markets-act.ec.europa.eu/gatekeepers_en

You can replace Gemini with Perplexity or whatever you want on your Android phone, that includes the main system-wide assistant.