Grin GUI wallet

Exactly. Grin is in the luxurious position to have two capable and fully independent node and wallet implementations which greatly reduces the chance of bugs being undetected.
Users can chose between them or use them both.

The plan (I can only speak for myself here) is to develop and maintain both as long as developers are willing and available and as long as the community supports their funding requests.

There is also space for new implementations like for example a Python or Haskell implementation of grin/mimblewimble.