>isn't anyone better off with a combo of eBay laptop + Debian?

No, not anyone. Hackers maybe, but not schools. First of all, you won't find 100,000+ cheap-but-quality laptops on ebay, not to mention you'll have to install Debian yourself. Not that it's difficult to install for "us," but these are targeted for school children. The whole point of this project is simplicity.

Any HD TV should work as a monitor, no? Presumably many people already own one. And USB mouse and keyboard are available dirt cheap.

It is intended to be connected to a television set via Composite or HDMI. I'd assume most families in the UK already have a television set so really the only additional cost is the keyboard+mouse.

The first thing that I ever thought when I saw the Raspberry Pi unit was how it would make a great device for MAME on a TV.

Hardware and firmware will vary across different laptops. This allows a stable platform for tinkering and experimentation.

I believe the intention is that if you break it, you can just turn it off and on and it's back to normal.

This was true of computers from the golden age of 80s UK home computers (where the R-Pi founders are coming from). It's not so much true of ebay laptops.

The BBC video is great. It reminds me of learning Microsoft "Extended Basic" on a Tandy Color Computer (aka, the CoCo) with a TV as a monitor around 1982-83.

Except back in the day, I had to deal with cassette tapes and only 4K (subsequently upgraded to 16K) of RAM.

30 years ago, I learned code by typing in hundreds of lines of (sometimes buggy) BASIC source code found in print magazines that came out only once a month. Today, all a youngster needs to do is a pull (and in just about any programming language) from Github. Progress is amazing!

Oh, but the things you can do without a monitor, a mouse and keyboard :)

OT: Can anyone give me a reason why this question has been downvoted? This seems like a valid, honest question.