You are probably right about this being a systematic failure but the fact that Trump wasn't the sitting President nor the nominee was a completely unique situation for the Secret service. Most past Presidents haven't been out & about like Trump nor were they campaigning this long & hard.The Secret Service would have been in somewhat uncharted territory and with the present political dispensation hostile to Trump, adequate resources were probably not sanctioned. Trump's team should have hired private security ages ago to help secure the gaps.
The failures of security in this case would not have been an isolated occurrence. There is a very high likelihood that this has happened before but without someone testing it like the shooter did in this case.
In most cases, it's the appearance of formidable security apparatus that dissuades somebody from trying their luck. That facade has been shattered. That's never good news.