May be I should not mention the word "insurance" as it has digressed to another subject.
Functionally, "Extended Service Agreement" is an equivalent of "Extended Warranty" but legally, they are different.
Some states demands that Tesla must legally be registered as an Insurance provider in order to be able to sell "Extended Warranty."
To bypass that legal terminology, Tesla calls that as an "Extended Service Agreement."
Whatever it is, it is still a tool if you want to buy or skip it according to your risk assessment.
As whether Tesla is reliable or not, Consumers Report says no.
If it is not, why Consumers Report also said 97% of owners are happy with Tesla?
That's because Tesla has serviced all problems for free during the warranty period and owners don't have to go to court to sue like GM 32-cent ignition switch.
As whether the repairs are affordable after 50,000 miles, here are a summary from Model S:
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Uncoiled | April 29, 2016
Windshield: $900
Windshield wiper motor: $500+
Headlight,: $1000 - has to be replaced as a next higher assembly even if just the bulb fails.
Side mirror: $390
MCU (17" touchscreen): $4,000
Door handle: $1000-$1200
Rotors, full set of 4: $1400
Sunroof seal: $1800(?)
Leaky cabin(?): $2500
Front end suspension work(?): ~$1000
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It's ridiculous that a burned light bulb replacement would cost $1,000 or a 12V battery that costs more than $300 but it seems to me, repair costs seem to be expensive.
By the way, it is true Tesla is a profit company so its programs including "Extended Service Agreement" is not a charity.
However, I do think that to have the ability to buy "Extended Service Agreement" is a privilege.
Consider those who do not have the privilege to purchase one:
1) Certified Pre-Owned: If you buy a Tesla directly from Tesla pre-owned selection, you are not allowed to buy Extended Service Agreement.
2) Third-party Pre-Owned: If you buy a used Tesla from non-Tesla source such as private owner or another company/dealership, you are not allowed to buy Extended Service Agreement either. However, if previous owner already bought it, you can have that owner to transfer that to you for a small fee.
So it is up to an individual to take in the data and decide for themselves.
And so far, from this forum, it is clear that there are 2 decisions after considering all the data:
1) No, it is not worth it.
2) Yes, it is worth it.