I think you're vastly undervaluing the cost of operating a auto shop + skilled labor. $700 was the cost for an auto repair business to 1) have a skilled technician find the problem 2) devise a repair method and 3) execute the repair.
Car repair is an expensive business to operate. Rent/building costs are high, lots of expensive equipment, one of the highest insurance rates around (you're lifting expensive heavy weights up and down over people all day) and labor is us usually relatively skilled and costly.
$50 for the repair and overhead ($5 supplies after markup, 1/3hr labor to include testing the system for leaks after the repair)
$200 for being the only people able and willing to do the repair.
$300 because they know how much the only available alternative costs.
1. invest money in a supply chain to have parts in stock for the battery assembly(take many months).
2. do a repair in Tesla warranty and pick up the Tab again if it breaks or worse the battery is damaged.
3. force a repair on the owner/lessee to pay for a new battery assembly which is less likely to have issues/safer
They already have this for other parts, it's trivial to extend the model to the battery. The problem is that the battery was not designed to be serviced without specialized tooling and expertise.
It doesn't have to be that way. It was their choice. They made that choice because it costs them less money to service the batteries in bulk at a specialized facility, at the expense of the customer. It's not like the customer has a choice. They already bought the car. What are they going to do?
Here is another option:
4. Design your vehicles in a way where a $1 part can be easily detached and replaced by anyone with basic tooling, instead of charging the customer $18k for a new battery.
This isn't just an issue with Tesla. This is an issue with almost all manufacturers today. They make more money when they replace the entire assembly.
Apple operates in the same way. If one of the keys breaks on your laptop, Apple will replace half the laptop. That's the only option. You can't replace the key. It was designed this way.
There are many repairs on existing cars that can be done incorrectly and lead to injury or worse. What are the extra guarantees that Tesla is providing for repairs done by them?
[1] https://twitter.com/Beastlyorion/status/1288242333864079360