Well, you need one if the instructor makes it a requirement, which my kid's high school does. They're actually tested on linear regression, but they don't know how to compute sum of least squares: they know how to type a list into their calculator and run the least squares function. It's on the exam, and if they don't have the TI graphing calculator with that exact function programmed into it, they fail the test.
Or, you know, not. It's a settled thing, and has been for a long time.
I'm too old to have been part of the TI cohort, but I had one of the early graphing calculators in high school (1987 or so), from Casio, and let me tell you having the ability to quickly graph a function is actually SUPER USEFUL, especially in an experimental/learning environment.
So maybe stop whining that "millions of kids" elsewhere or in the past didn't have one, because it's completely irrelevant.
https://www.amazon.co.uk/FX-991EX-Advanced-Scientific-Calcul... (£20)
Pretty sure most of my physics degree class used an FX-83.
I'd LOVE for your kid's 0 test score to go viral on social media and get kilobucks of revenue from the 15 minutes of fame to pay for college.
For calculus, that's not so good. To have any intuition about calculus you need to be really proficient at algebra and graphing--by hand.
When you apply calculus in engineering and physics, sure, use the calculator. But when it's just calc, it really needs to be structured to be able to be done by hand.