If Google had made the services modular, it would probably be easier to replace them with homegrown ones, but they made it a blob because that gives them more power. There are efforts to replace this blob with other manufacturers' blobs (like the one Yandex is working on, or microG) but it looks like this is not easy and it's not going quickly.
We also probably get a typical reverse-engineering situation where Google's services will always be several steps ahead of anything anyone else can offer as a drop-in replacement, so the alternative looks less and less enticing to the average consumer and they'll probably go with the blob :(
but they made it a blob because that gives them more power
i guess you're talking about the Google Play Services library, which is updated independently of my app. or something like that.
not quite sure. trying to clarify this for my own understanding.