You know, I was assuming the 15th century actually meant 15XX in this case because of the way it followed an already established point in 1491, but then further digging has Ed Greenwood saying it's nebulous but right around 1490, so I will say that's what I get for using the internet as a source.
This then leaves us with anywhere from 0 to 90 years for the bandits to move in, which is fairly useless for determining how dangerous that place is. Maybe someone really into the lore can clarify, I don't even own the adventure. I just know it's the 5e generic small town for many new players, and it's not described as dangerous as some here are saying all undefended small towns on the frontier are.
Well, I lack reading comprehension, but doing a little basic research I found this "Iarno was initially a member of the Lords' Alliance, who was sent to the town of Phandalin to establish constabulary (as the town did not have a powerful authority).[5]
1491 DR,[note 1] Iarno broke ties with the Lords' Alliance and created the Redbrands under the guidance of Nezznar."
So, the Red Brands seemed to have moved in right around 1491, since we have an established date for their formation. Oh, and Nezznar was a drow, the main threat behind the adventure, this little tidbit is fun
"
Circa 1491 DR,[note 1] the drow promised a share to Iarno Albrek of the Lords' Alliance in exchange for his loyalty and aid. Nezznar was the true leader of the Redbrands, controlling them (as well as the Cragmaw tribe of goblinoids) in the hopes of finding Wave Echo Cave and unlocking its secrets (while also keeping it a secret for himself).[1]
After finding the mine, Nezznar came across two of the Rockseeker brothers, Nundro and Tharden. He slew Tharden, but kept Nundro as a hostage, as he thought the dwarf knew more than the he was letting on. From then on, he focused on getting to the legendary forge, but was set back by hordes of undead, and rooms full of poisonous fungi.[1]"
So, to reiterate.
Phandalin was destroyed by orcs. Then five hundred years later settlers went to the ruins to clear it out and rebuild.
The same year the town was "well-established" a Drow turned the wizard who was sent as a Constable, helping him form a gang of brigands to terrorize the town, including a Nothic. The Drow also controlled a nearby tribe of goblinoids, lead by King Grol a Bugbear in Cragmaw castle, who had enough forces to send his Lieutenant Klarg, a hobgoblin to run an outpost near the town. So, easily thirty to fifty goblinoids without checking the adventure.
This drow and his squad, which this drow also controlled a massive spy ring, as well as his pet Giant spiders, were busy in the cave because of hordes of undead.
This adventure also features a nearby banshee, a village full of blights and possibly zombies, and a young dragon. Oh yeah, there is another nearby ruined village in Thundertree, and the mine the Drow is in was destroyed and abandoned, and there was a gathering of orc forces at Wyvern Tor....
But yeah, this just proves Faerun is totally safe and you don't need to worry about being attacked by anything. I mean, within a few years of being resettled there were only... five diffferent threats that could have destroyed the town, between the bandits, the goblins, the drow, the dragon and the orcs?
