I wouldn't call it a mistake, more like being caught between a rock and a hard place, where Android basically forced them to give up on SMS support even though they'd have liked to keep it: https://community.signalusers.org/t/signal-blog-removing-sms-support-from-signal-android-very-soon/47954/57
But yes, it was really nice when I could use it as my SMS app. Then again, very few people in my country use SMS in the first place - it's all WhatsApp, and it was never able to have support for that. Luckily, most of my friends have adopted Signal by now.