Fines are meant as a warning. If you impose a 50bn fine then every other company out there will start having second thoughts about entering the EU market. Lobbying will skyrocket and a lot of things could stale or worsen for the economy as a whole.
Yes, don't enter the EU market if you want to dodge taxes. More countries should be enforcing this..
Fines are a deterrent and punitive measure. They punish you for doing something and make you hesitate to do it again. If these 2 goals aren't reached than the fine isn't really effective.
If they were a warning then you'd expect the second offense to be punished by some action that can make the fine feel like "just" a warning. What's that action? How effective is a string of endless warnings that are never followed up with real action?
One way would be to implement a series of exponentially growing fines. This would make them less "cost of doing business" and more "massive liability".