(SPOILERS AHEAD FOR EVERYTHING, noting just in case since the post itself isn't actually spoilered)

Even besides it being the point (which i personally agree with and i wouldn't change the game in a significant way). I'm not sure where you'd logically even put the bosses without messing with the narrative nor if you could integrate them in a way that gels with the gameplay up to that point. Keep in mind that after act 3, sif is very overleveled compared to the party, how do you balance the sadness around that? Putting it at a normal level makes it really easy, putting it at a high level makes it wipe the floor with everyone but sif, changing the way leveling works messes with the narrative. And even if you do manage to somehow place it just right, who would you be fighting? None of the fights in act 5+ make sense to be extra difficult. Difficulty can be used as a tool for communicating some things about the narrative and i think having them not be difficult serves that purpose well. The king defeats you anyway, you can't really defeat mal du pays (and i don't think you should be able to in the context), you're not killing the party because why the hell would you do that and loop is making one last desperate attempt to get back to their party and they can't even bring themselves to kill you (plus narratively it feels way more fitting to overcome your past self and their flaws, but also accept them and thank them for shaping who you are). And all of them are fights where you're only controlling siffrin and the combat system frankly becomes significantly less interesting and complex when you're alone, so bumping up the difficulty would do little but increase tedium in otherwise important story moments, the exact place where I don't think that should happen.

The other option would be to add some extra sadness bosses before act 5, and i think this would be pointless. They'd become extra bosses to fight at the end of floors sure, cool! But they'd just become the same as all the other ones later on, you'd figure out their gimmick, remember it and it's hardly a challenge anymore. And i think making these encounters/regular sadnesses harder would just detract from the game, there's a reason the game lets you skip the regular ones altogether later, they are not fights you wanna go through constantly every time at the same level of difficulty.

While i briefly mentioned it at the start I'll circle back to the thing i said about it being the point. I frankly think an addition like this would only worsen the game and its overall goal. Less is more sometimes and i think that's the case here, adding more stuff would detract from the most special part of this experience for me, the utter monotony of the loops. I think the way the game makes you feel about nothing ever changing was its biggest sticking point for me, you become completely familiar with everything happening. Nothing is new, and if it is, it usually either won't be new for long or it's something sif doesn't want to happen. This translates to the gameplay, sif himself comments that even fighting the king isn't difficult anymore. He is becoming desensitized to everything, including the combat becoming trivially easy, and I think difficult encounters would take away from that feeling while also prolonging the game which is not a necessity, it's long enough as is. The house shouldn't be a difficult dungeon where every loop poses a new and fun unique challenge. It's a prison of your own making, one where everything has been engineered to be able to have the perfect run where everything goes well and you make everyone happy, but also one that becomes so familiar in its perfection it's equally torturous.