The reality is that both are true.
There are multiple positive network effects at work, as it's evidenced by the growth of these companies.
However the barrier to entry is super low, so competition can be quite high. Like you said, just by subsidizing rides/drivers you will start gaining some market share.
In the end it's all gonna be about who has the most cash and whose efficiency is the highest. Yeah you could hire a promoter and give out flyers outside a venue, but if you check the data, the ROI is abysmal.
This is bad for customers.
It's pretty easy to have a network that's not very big that always has someone at the airport, or always is available for trips that stay within a specific neighborhood, or can always be arranged with a little more advance notice, or whatever.
And the relevant part of pointing this out is that as long as that's true, which is forever, then these large companies will never be able to extract monopoly pricing.
As soon as they start gouging customers people will immediately flood in to siphon off the lowest hanging fruit or easiest segments of their customer base.
That dynamic will never go away, it's not at all clear what the point of all this buying of market share is, given that it will never be truly defendable.
To start a new ridesharing company all you have to do is attack and carve off the most profitable element of the big company's product mix.
All you need is to undercut them in certain high profitability routes, like airport runs, or luxury vehicles for executives, or anything, and you have a toehold.
There are some positive network effects, but there are also negative network effects. Having more users doesn't mean you always win as the big company.
For example if you have no customers all you have to do is be outside of a stadium at the end of a show, or waiting by an airport during a busy landing period, and have rudimentary abilities to let people know you exist, and you'll get defectors.