My 2015 MBP bricked when upgrading to Catalina. Thankfully I managed to recover back to High Sierra via Disc Utility/Recovery Mode. I upgraded to Mojave and now

My 2015 MBP bricked when upgrading to Catalina. Thankfully I managed to recover back to High Sierra via Disc Utility/Recovery Mode. I upgraded to Mojave and now refuse to upgrade it any further. I have an iMac which will be staying on Mojave too. I worry that this opens me up to security risks, but the reality is, upgrading could break it again and force me to replace it when the hardware still has plenty of life in it.