That’s like saying there’s nothing wrong with Word because you could just build your own version. Adobe controlled the format and since they didn’t care about quality, performance, security, or tool quality that doomed it.
The other big thing which hit Flash was mobile. Rearchitecting it to support the equivalent of the web’s responsiveness was a huge problem which got lost in the “Steve Jobs killed Flash” narrative. Battery life wasn’t the only thing which made it unpopular on mobile – that could have been improved even though some aspects of the platform made that hard – but also the fact that Flash was based around mice and fixed-size displays. Very little of the existing content worked well (often at all) on the mobile devices which did have Flash.
Vector graphics designed for a desktop display or a printed page generally do not work well on small displays unless scaling to fit or scrolling is acceptable. I can hardly imagine what one would do to make something like SVG reflow in a sensible way. Line of business (web) applications often have the same problem, i.e. there is often a display size below which they simply are not usable, and that should be no surprise to anyone.
The other big thing which hit Flash was mobile. Rearchitecting it to support the equivalent of the web’s responsiveness was a huge problem which got lost in the “Steve Jobs killed Flash” narrative. Battery life wasn’t the only thing which made it unpopular on mobile – that could have been improved even though some aspects of the platform made that hard – but also the fact that Flash was based around mice and fixed-size displays. Very little of the existing content worked well (often at all) on the mobile devices which did have Flash.