NOTE: I didn't check his math, but no one on Reditt seems to have questioned the math, so I'm gonna accept it unless someone wants to take the time and either verify or refute it.
So to get to level 300, where you start to get close to maxing out everything on your character - assuming that XP is determined similar to Skyrim, it would take 2,287 hours at an average of 500 XP per hour.
At an average of 4 hours per day, that's 571.75 days. That's 1 year and nearly 7 months. So over a year and a half, playing Fallout 4 for 4 hours a day. And you are worried about becoming a "Jack-of-all-Trades"? If someone plays the same character for a year and a half, at 4 hours a day, they *deserve* to be a "Jack-of-all-Trades"!
At even an average of 10 hours a day, it would take you about 8 months to max out the character. I dunno about the play-style of most people, but I doubt that most players play 10 hours a day - and even those that do - 8 months of the same character is quite the commitment (with no starting over, trying out new characters, new builds, maybe starting over to add in new mods, etc). And for those that play only 4 hours a day, a year and a half on one character? No starting over to add in new mods? Try a new build? Choose a different direction? That's a LONG time and some *serious* dedication!
Maybe the math is wrong. Maybe the assumptions are wrong. Maybe you level faster. I have no clue, and neither does anyone else outside of Bethesda that I am aware of - so these numbers are just as good as any for now. And even if it takes half the time we are assuming based on these numbers, that's still over 9 months for the 4 hour a day player.
And, there are always Mods.