You're right. In the very early 80's, the issue facing computing wasn't so much size as cost.
By the time the Spectrum launched, the Atari 8-bit line had been around for 2.5 years, the TRS-80 Color Computer for 18 months, the VIC-20 for a year or so, and other too.
All of those offered colour graphics, some quite limited, but in the case of the Atari, the palette went up to 256 colours, They also offered multiple sound channels.
On paper, they all blew the ZX80/81 away, and should have been significant rivals to the Spectrum. Some (especially the Atari) were theoretically in a different league altogether.
But the issue they had was cost. All of the competitors represented a massive outlay, whereas the Spectrum, whilst not exactly launching at pocket money prices, was something that could be put on a Christmas list.
This was Clive's expertise. He never strived to make the best systems full stop, but he did know how to make a good system on a budget.