I managed almost 15 Campaigns in a row (without dying/ failing), and they never increased in difficulty, just reset back to the usual 3 non-alerted missions with 1-2 targets.
I wouldn’t call Prestige a “reward.” Prestige was clearly implemented to help speed up the latter half of Mastery by giving the player more XP and more Merces to get useful items faster. This was in direct response to just how grindy and reptitive Freelancer was/ is.
But yes, unlocking more equipment slots, shed poison, garage bomb, and grapes makes it easier to “start over.”
And, I actually really enjoy the Safe House! In fact, it’s practically the only reason I’ve played Freelancer as much as I have. It’s no secret that many diehard Hitman players enjoy the dress-up and role-play that’s become associated with the game, and I adore IOI for recognizing that and crafting a truly wonderful Safe House for us 
Across the series, I have 420 hours in H1, 37 hours in H2, and 391 hours in H3. That’s approx. 850 hours spread across 7 years. That averages around 30 minutes a day or 2.5 hours per week (obviously, that’s not consecutive).
I am currently at Mastery 92, have completed 87 showdowns, and killed 868 total targets. Before prestige, I had 3 million Merces. I would say I have put about 80 hours into Freelancer at this point (less than 10% of my time across the series). For many players, that is a lot of time! And, there are also many more players with well over 1000 hours across the entire trilogy.
I would say around Mastery 50 (approx. 30 hours in) is when the Grind of Freelancer truly gets bad. Before Mastery 50, safe house unlocks are pretty consistent! Through a campaign, you’ll get several! However, around Mastery 90, you’re lucky to get even one Mastery Unlock in a Campaign (even with the pittance that the newly added prestige offers).
The average AAA single-player campaign is about 6-12 hours of content. Perhaps you think 40+ hours is more than Freelancer deserves. And I agree! In my 80 hours, I haven’t achieved Mastery 100, Employee of the Month, or even got halfway to GOAT. Now, these are obviously “end-game” achievements for die-hard, committed players. And maybe there’s something wrong with enjoying Hitman that much.
But I don’t personally think Hitman is “any other video game.” I adore these games. I will happily sing their praises day-in and day-out. I rarely look at upcoming games, but when the new 007 game comes out, I’ll buy it and play it! I have never had more fun in a video game than playing Hitman 2016 for the first time. And while I definitely have my favorite missions/ maps, I genuinely adore and admire the entirety of the mission stories, from France to Romania. The level design, the detailed and fleshed-out targets, the dense lore and dialogue, and the wonderfully comedic supporting characters make the WoA trilogy truly unique and exceptional!
And this is why I seem so critical of Freelancer/ Hitman. If I didn’t care about Hitman, I wouldn’t post 1000+ words of critique and conversation about just one of its several game modes.
I am critical because I love and adore these games. And I think Hitman and IOI can do better. I want them to do better! I want WoA to be its “best self!” In my experience, Freelancer simply doesn’t live up to what WoA has offered previously or what IOI is capable of.
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The safe house is really cool though 
Massive props to the level designers, they consistently knock it out of the park 