I'm sure you could have replaced the chip containing the ROM and replace it with your own version. Things were a little bit more rough back then.
I made the ROM writable by soldering a CMOS ram chip across the ROM (was it 16KB?) and any write to the ROM would end up in the RAM. A switch selected either the ROM or RAM to read from. On startup simply copy the ROM to the RAM and flip the switch.
From there on you can read/write the original ROM and modify it to your hearts content.
[All prices from Wikipedia]
But they both absolutely crushed the quality of almost any keyboard you get on a computer today.
But it was double the price, putting it out of reach for many more families than the Spectrum. This made a kind of critical mass that lead to a huge community around the spectrum in the UK. And a fantastic burst of creativity as the barrier to entry to writing video games dropped as low as it would ever get.
Perhaps the best thing about the Spectrum was that the manual, which documented the built in BASIC programming language and also the Z80 asm opcodes.
Edit: I should probably steer clear of culture war topics but let's be honest - the designers cut every corner they could think of and consequently the Spectrum looked and sounded atrocious compared to the luxurious C64. But I probably do owe my tech career to one.
The Speccy owners tried to prove otherwise during countless arguments in the schoolyard, but deep down they knew.
I used the C64 only for games, and the spectrum for my own programs and the odd top game
and yes I was on both sides in the playground,
but the C64 had something missing that the spectrum had, hard to put a finger on it, the spectrum did have inferior games (based on the specifications), but It was more accessable or hackable (in todays speak)
I'd say the spectrum kids was more into modding it, typing their own code from magazines and learning stuff, but the C64 kids used it more like console. and could not see the reason for having a spectrum (due to the inferior spec etc..)
But all machines of that era were so low level with plenty of tricks to exploit.
maybe it was the school I went to and the information the collection of kids had
The C64 was a great music-synth with the capability to do 1 certain type of game kside scrollers) in 16 shades of brown vut that was it.
The sheer endless variety of games the Speccy supported knocked the C64 into a cocked hat.
I do find the Speccy charming though!
Back then, C64 vs Spectrum discussions were like vi vs Emacs. Good old fashioned tribalism is a thing of all ages.
It was never really an open system. The firmware was closed source (you could eventually buy an unofficial disassembly) but you couldn't update it with your own changes. You could add external hardware, with some effort, but it was never trying to compete with a backplane system like S-100.
What it did have - apart from very low initial cost and wide availability - was instant-on (no formatting, no setting up, no downloading and installing), staged progress from beginner to expert (i.e. from BASIC to assembler), a small comprehensible system (the manual explained everything BASIC could do, assembler needed an extra book or two but was still very manageable) a very large market with very low cost of entry and high margins (you could write a game, advertise it for almost no money, and send out the initial copies from home with no duplication costs), and a supportive culture of print magazines, occasional TV news and documentaries, and computer shows. (Face to face clubs were much rarer and less influential.)
The physical case design was just plain cool. And it worked with your own TV, which was almost like being on TV.
So basically a combination of instant gratification, tiny-system simplicity with tough constraints that rewarded clever solutions and creativity, low-friction financial opportunity, cultural reinforcement and support among peers working at a similar level, distinctive aesthetics with a coolness of a sort, and affordability.
I don't think it's a coincidence that it happened not long after punk and synth pop in music. Even if their output was very different, all of of those had very similar cultural features.