I mean, redundancy and fault tolerance aren't absolutely mandatory for a design to be good, as long as it's a simple design used in design operating conditions with a low chance of failing. SRBs flown pre-STS-51L in design temps didn't have signs of burn through, only ones flown cold showed signs (which the fact those flown out of spec showed signs even before 51L should have set off alarm bells and gotten the situation resolved without loss of life)

But yes, it definitely was made a better design by adding that fault tolerance, and without adding too terribly much mass and complexity. And it makes sense that they'd add it in after experiencing that failure mode (even if it was caused by out of spec operation) just to give more peace of mind.