In any case, the expected growth in the smallsat market is significant and the market might exceed the "traditional" satellite market by 2025 [1].
This leaves plenty of room for "boutique" launch services, especially considering that SpaceX will be their own best customer with their StarLink launch campaign.
So while they might be cheaper as far as $/kg is concerned, other factors (launch window, target orbit, payload integration, logistics, ...) have to be considered as well and the bigger the market, the more room for competition.
I'm not saying that no one will use (SpaceX) rideshare services - I'm just pointing out that SpaceX won't be the only reasonable option since $/kg isn't the only relevant metric to consider.
[1] https://www.satellitetoday.com/business/2018/10/22/just-a-sm...