I don't ignore Boston's young players, but Boston is not a team constructed for long term success as it has no go to offensive player, no great take it to the hole players, lacks a reliable post game, has no rim protector, and has a fair amount of inefficient undersized chuckers on the wings. Nothing Boston did in the offseason resolved the team's deficiencies, though Amir is at least a solid interior defender (though not a rim protector) and Lee may still have a bit left for the offensive post game.
Toronto is just a better constructed team with more established and better players. Lowry is significantly better than Smart and Thomas (now if you combined Smart and Thomas, you would get Lowry). DeRozan is a much better all around offensive player than Bradley (though obviously Bradley is a better defender). Carroll is a significantly better version of Crowder, better defender, better/more consistent outside shooter, better ball handler. Patterson is pretty much Sullinger (worse rebounder but better shooter). Valanciunas is everything we want Zeller to be, but never will be. Amir and Lee are certainly better than Biyombo and Scola. Turner is a better all around offensive player than Joseph and Ross, but isn't as good a defender or outside shooter.
Starters big edge to Toronto. Rotation/Bench each team has flaws and I'd call them about the same. Toronto will win the Atlantic again and it won't be close. That said, neither team is a real contender as neither has elite championship talent on the level of the real contenders, like the Cavs, Warriors, Spurs, Thunder, etc.
If you think IT is not a go to scorer or a player that can take it to the hole, then Toronto is also lacking those players. How are Lowry and Derozan go to scorers/players who can take it to the hole if IT isn't? IT is a better scorer than any player on Toronto, and he's better at drawing fouls/driving to the rim than Lowry and Derozan. Toronto also lacks a rim protector, especially now that they lost their best interior defender (sorry Biyombo likely isn't changing anything for any team significantly).
It's too simple to say IT + Smart = Lowry. That's not remotely true, imo. Lowry isn't that good of a player. He's a
nice player who is a solid/two-way guard, but he's not elite at anything. IT is actually a nearly elite volume scorer based on effiency/rate and other metrics. Smart is an elite defender or at least projects to be one (Smart as a rookie already had a higher DBPM than Lowry has ever had in any single season). Combining Smart and IT would result in something much, much, better than Kyle Lowry.
Boston has inefficient wing players, I totally agree, but so does Toronto, and the difference is our inefficient wing players aren't overshooting like Demar is. Demar isn't that great of an all around player. He's not a good defender, his mid-range shot is "his game," but it is worse than Avery Bradley's, and he can't shoot a 3 worth a lick. The only reason he even approaches mediocrity in efficiency is because he can draw fouls like a madman, which is a valuable quality I admit and something I wish our wings had.
Talent wise the biggest thing Toronto has on Boston is Val/a consistent post scorer. I totally agree with that statement. Unfortunately for them, their coach is too dumb (talk to Toronto fans) to properly utilize that talent so it almost becomes a moot point.
The Celtics also have better defensive talent from top to bottom.
So of your 5 points in your first paragraph only one of them is really true relative to Toronto's roster (post scorer).
Don't get me wrong, I think there is a good chance Toronto continues it's success and remains a 47-50 win team, but it is far from a guarantee. That's all I'm arguing. It's also far from a guarantee that they will be way ahead of Boston. You are overrating their talent.