So this gets me wondering, What happens when the block rewards are too low to sustain mining?
There are several topics on this, in case you're interested. Short version: I don't worry about it
![Smiley](https://bitcointalk.org/Smileys/default/smiley.gif)
1. Won't this lead to problems on the bitcoin network? Like slow transactions and high fee?
Transaction speed is defined by the protocol, it's the same (on average) with 1 miner as it will be with a million miners.
Fees depend on block scarcity, at the end of 2017 fees often got half as high as the block reward already.
2. Is there a contingency plan for this kind of scenario?
Yes: fees.
If anything, lower block earnings will lead to a lower energy consumption and a lower drain of resources from the Bitcoin ecosystem to hardware manufacturers and power companies. But high block earnings lead to a high difficulty, and thus secure network.