Wholeheartedly support this idea.
Despite the expenses of games, I think many developers can make functioning multiplayer bots, and Halo isn't really a complicated game to begin with (simple, but hard to master). Heck, games like ArmA and Red Orchestra 2 (despite being a PC game to begin with), had pretty good bots to fill the void and they are really complicated games.
The old Battlefield titles from 1942 to 2142 had multiplayer bots that can be played offline and they had some pretty good bots. I was pretty surprised in 1942 where the bots would actually control the gunner positions in maps like "Battle of Britain". I don't have much interest in the new additions of the series because they are more like "touch and go" kind of games where they exist as long as the hype is there.
The Gears series (minus 1 and the friggin' remake) had multiplayer bots even though, being a console game. It has an intricate movement system which I find pretty surprising how well the AI navigates the map.
The introductions of multiplayer AI in Call of Duty: Black Ops was great news to me, which meant that I can play the multiplayer alone at my leisure, even when the internet is down. MW, WaW, and MW2 did not have bots, but when the community made mods for them, I find myself playing these titles again.
Titanfall suffered because it was a lackluster game to begin with and I believe, if the game had Titan/Pilot bots for offline (not including the survival mode, but modes like attrition), Titanfall 1 would probably have a population bigger than 50. Titanfall 1 is nothing more than digital junk collecting dust in my Origin library and it's a real shame.
Halo doesn't have that problem because it has a -Yoink- campaign that can be played with harder difficulties and even harder with skulls (sometimes Firefight). However, even if the campaign is memorable and replayable, multiplayer is another major component of the Halo franchise that makes the game whole (50% campaign/ 50% multiplayer). The lack of experiencing the Multiplayer at anytime makes Halo feel half full and I think Halo MC collection on the PC would be the best time to implement offline multiplayer bots, starting with HALO 3.
Black Ops 4 blew my mind with the introduction of bots for Zombies, and therefore, 343 should implement allied bots for Firefight (kinda feels a bit too lonely when lone wolfing Firefight in ODST/Reach)
The point is, bots are essential to preserve the game's longevity, or at most, the experience of playing multiplayer at any time (even when the internet is down or slow).