If you accept, they'll never ask you again. You're only option to reject is "not now".
Another problem is a missing alternative to Google Mobile Services (GMS), which are installed on virtually all android devices. For example, if you need push notifications or in-app purchases you are dependent on them as well.
IMO, it is the widespread distribution of GMS (and a lack of alternatives thereof) that make google a quasi monopoly.
Don't Amazon, Samsung and Huawei have their Android app stores, for any app?
This means, that everyone will still use Google Play for in-app purchases out of convenience.
Amazon Device Messaging: https://developer.amazon.com/docs/adm/overview.html
Huawei Mobile Services: https://developer.huawei.com/consumer/en/hms
A service like GMS would be important for Samsung, too, as there are quite some Samsung devices out there. However, with GMS pre-installed on all Samsung devices, the Samsung Galaxy Store is completely useless as everybody and his brother will use Google Play for app and in-app purchases.