Google is a business like any other - and when they have the power to lever a monopolized situation - they will.
For example, they may start integrating technologies for which they have exclusive, or at least 'special' access. Can you imagine if all of a sudden Google apps start performing better than anyone else's?
Or what if they integrate technology that de-facto collects usage and behaviour - even from other domains - and then lever that competitively?
The power that Google already has, relatively unregulated is crazy. Remember that Google is the company that could change the outcome of elections ... possibly without us even knowing. They could drive markets up or down at will.
This would probably happen as a 'slow drip' - not one step being close enough to consumer awareness to create problems, and what with 'business friendly' politicians, nary a worry of legislation getting in the way.
They could introduce these technologies under the guise of 'improving user experience' (and maybe legitimately so to start), but other PM's, new CEO's etc. just take the opportunity before them. Why wouldn't they?
We worry a lot about 'Net Neutrality' at the network level, like it's a religion ... but that we don't worry about 'information neutrality' as well I find quite bizarre.
I suggest that having '1 major provider' may be a problem, doubly so if it's an entity like Google, and that frankly we don't really gain much from this fact at all. If Mozilla were the provider, great. Even Apple might be a better choice simply because they are not in the business of managing our information - at least right now, for them, it's a headache, not a revenue source.
So the concern is real.