Actually, Apple's sales figures for the first quarter of this year are the highest they have ever been for any first quarter to date.
https://www.cnbc.com/2018/02/01/apple-earnings-q1-2018.html
https://www.macworld.co.uk/news/apple/apple-financial-results-3581769/
And specifically iPad sales went up from last year's first quarter. It's also of note, that Apple's iPad sales figures were the highest overall in 2014. And declined from there. Coming up at 4 year mark its easy to assume that many of those that bought iPads in 2014 are just now coming back to renew their devices. 4 years is about the expected turn around for tablet devices. So the decline in sales in the last 4 years matches up with how mobile devices are bought and how long they are kept. I suspect iPad sales will increase this year because of that. It's all about cycles.
And yes most other tablets that actually support mice aren't actually flying off the shelves. Microsoft has had a really tough time pushing their Surface tablets which incidentally do support mice and many times are much much more expensive than iPads. Also Microsoft seem to be moving towards the mouseless experience on tablets with their new Windows 10 S Mode OS for tablets. While its still supported sure, the focus is on touch screen interface, rather than mouse interface.
Apple still dominates tablet sales. Tablet Wars: After Apple iPad, Amazon Is New No. 2 | Investopedia
and the recently released entry level iPad at $299 is the one to be keeping an eye out for. We'll see how well it does at the end of the second quarter of 2018.
Also, the author of that article really has a chip on his shoulder for Apple. Yes tablets break, but I don't see Android or Windows tablets being offered at schools with rugged cases either. i.e The protection aspect has always been up to the customer. Only Amazon and if you want, Leapfrog have ever offered tablets with included rugged cases for children. The lack of storage for the Pencil is a valid concern. But seeing as its not an actual part of the iPad, unlike integrated stylii for other tablets, its hard to really give them too much grief for not adding a place to put it on the iPad itself.
Maybe offering a magnetic case for the pencil that attaches to the back of the iPad may be a good idea.