The line manager's voice
is often the only signal upper management gets about tech debt/infra/security...until it's too late.
People say they can't get upper management to pay attention to their desperate pleas. This is true; it can be hard...the first time.
When they deny your request for eng time, patiently describe some of the things you are afraid will happen. Be friendly! In writing, with witnesses on carbon copy.
Then follow up. Respectfully bemoan that you weren't able to allocate $x hours to prevent the problem which will now take $xx hours recovery. Point out another thing that may fail, ask for $x hours to fix it.
Repeat as needed.