This turned out to be an innocuous threat, though it was definitely a threat (the security perimeter around the president was breached). Expect more such threats to hit the news: Trump is a uniquely unpopular president (and rightly so), therefore there’s a uniquely great motive for actions against him.
There are no perfect security measures; all measures have their holes and oversights. The Secret Service knows this, and their response to it is redundancy. They have multiple shells of security surrounding the president. Each shell is imperfect (see my first point), thus has a slight chance of being breached. However, the overall chance of breaching all the shells is vanishingly low.
George W. Bush was in his time astoundingly unpopular, as well, yet he served out both his full terms. The Secret Service successfully stopped all threats against him. The strategy of having multiple security layers worked.
And yes, there were foiled assassination plots. Such plots have existed against all modern presidents. The reason you haven’t heard of them is that the Secret Service lives up to its name: they deliberately keep news of foiled plots classified so as not to have the news of them inspire copycats.
That policy only works if the outermost layers enable plots to be stopped before they start. If surveillance allows a plot to be broken up before anyone can be deployed on the ground, or an agent arrests a gunman well before he gets within even a mile of a motorcade route, it can all be easily hushed up.
It’s basically impossible to hush things up once the outer layers get penetrated. The president is accompanied by a retinue of reporters wherever he travels, and it is newsworthy when the security perimeter gets breached.
That’s what made the Greenpeace action so successful: once that guy got through, it was virtually ensured there would be multiple videos and reports of his action hitting the news. It also made it risky, of course: he could easily have been shot out of the air and killed.
But I am digressing. Go back to my initial points about all security measures having their holes, and Trump being uniquely unpopular. Put those two together and it means that the chance of the outer layers getting penetrated is much higher than for most presidents, simply because there’s more people attempting to penetrate them.
This time, it was a harmless penetration. The Secret Service may well have even been aware that Greenpeace might be planning such an action, and have decided to deprioritize it in favor of focusing on more serious threats. Greenpeace, after all, has a long, proud tradition of confining itself to only nonviolent direct actions, so the worst that would likely happen is what did in fact happen: an embarrassing penetration, but no actual harm done to anybody.
The next time, however, the penetration might not be so innocuous. It probably still won’t be successful in physically harming Trump (odds favor the inner layers of security working), but the news will be more dramatic and dire than it was this time. Also, keep in mind the copycat effect: now that there’s been one such story hitting the news, it will probably inspire others to make their own attempts.
So, this is probably only the beginning.