Is this right use case to replace IAP with apigee-X ? else what's other design considered need to be considered ..
What? No!
IAP has a good use case - when you can issue identities from Google cloud for access to things in Google Cloud, use IAP. Super simple.
Apigee helps you with a more broadly construed API management. A variety of different consumers. Authenticating both the app and the person - effectively TWO identities. Implementing security mediation for things that are not google cloud native. Advanced analytics and monitoring. Monetization. Subsetting APIs into products = The API Product concept. None of this is possible with IAP.