The thing about Alolan Raichu is that it's only good while paired with Tapu Koko, much like Kingdra is only particularly good when paired with a rain setter; however, Tapu Koko is so good in the meta right now that it's not really a constraint on building. If I want to use Kingdra, I basically now have to work under the constraints of using Rain, which is a given for any playstyle, but Raichu's only requirement is "use one of the best mons in the game", so many teams have no trouble fitting him and a lot would probably already include Tapu Koko. It's not competing for the team slot the same way you're not weighing Kingdra vs Pelipper; the point is the two being a core that work better together.
Besides that, the weather comparison is lost a bit since in prior generations, Politoed's only real support was the rain itself and very generic stuff like Toxic, with the Rain sweepers springboarding from there and not looking back. Pelipper is such a breath of air for the Rain archetype because it's a Rain setter that offers a decent amount of support besides the actual rain, particularly Pivoting to bring in the Rain sweepers without loss of momentum. Tapu Koko is not used as a Terrain setter, rather he is a very good mon who includes Electric Terrain in what he can bring to a team.
I don't see how sharing the role is an automatic strike against Raichu with Koko. Double type cores to chew on checks has been a concept since Generation 4, and it's not even like the core is totally redundant on synergy, as Raichu can lure Bulky Ground types with Surf and Grass Knot (frees up the Hidden Power slot on Koko) while having a means to break the occasional Poison type like Mega Venusaur and Amoongus, two mons that Koko has to go mixed to deal with and (as bulky Grass types) would give the rain archetype trouble. And to note, Alolan Marowak (tank set at least) has to be kept healthy to check Surf sets without falling to kill range when looking at the core used outside of Rain, something noteworthy given Alolan-Marowak is such a strong mon for checking Electrics as well as in general.
252 SpA Life Orb Raichu-Alola Psychic vs. 232 HP / 4 SpD Venusaur-Mega: 265-312 (73.8 - 86.9%) -- guaranteed 2HKO after Stealth Rock
252 SpA Life Orb Raichu-Alola Psychic vs. 248 HP / 144 SpD Venusaur-Mega: 237-281 (65.2 - 77.4%) -- guaranteed 2HKO after Stealth Rock
252 SpA Life Orb Raichu-Alola Psychic vs. 248 HP / 92 SpD Amoonguss: 330-393 (76.5 - 91.1%) -- 18.8% chance to OHKO after Stealth Rock
252 SpA Life Orb Raichu-Alola Surf vs. 252 HP / 112 SpD Marowak-Alola: 218-257 (67.2 - 79.3%) -- 37.5% chance to OHKO after Stealth Rock
Besides this, it should be noted that Raichu isn't totally neutered if the Terrain is overwritten, compared to Rain sweepers losing a LOT of their momentum if the weather wears out or is switched, thanks to Raichu having a fairly competent speed tier and outspeeding the other Tapus. Worst case scenario, he can Volt Switch out to maintain momentum, but he actually leaves a decent dent in Fini and Bulu, while having enough presence that Lele can't swap in unaided at least.
252 SpA Life Orb Raichu-Alola Psychic vs. 164 HP / 0 SpD Tapu Bulu: 161-191 (50 - 59.3%) -- guaranteed 2HKO after Stealth Rock, Leftovers recovery, and Grassy Terrain recovery
252 SpA Life Orb Raichu-Alola Thunderbolt vs. 252 HP / 0 SpD Tapu Fini: 244-291 (70.9 - 84.5%) -- guaranteed 2HKO after Stealth Rock
252+ SpA Life Orb Tapu Lele Moonblast vs. 0 HP / 0 SpD Raichu-Alola: 253-300 (96.9 - 114.9%) -- 81.3% chance to OHKO
252 SpA Life Orb Raichu-Alola Thunderbolt vs. 0 HP / 0 SpD Tapu Lele: 138-164 (49.1 - 58.3%) -- guaranteed 2HKO after Stealth Rock
The relationship between Tapu Koko and Raichu seems less like the Rain archetype, and more like the point in XY where "Sand Teams" were "Tyranitar and an Excadrill to sweep". Fast, Terrain powered Electric types are a task to wall, especially given some of the mons picking up for defensive prowess are things that don't fare well against Electric types, like Celesteela and Toxapex
252 SpA Life Orb Raichu-Alola Thunderbolt vs. 252 HP / 252+ SpD Celesteela: 221-265 (55.5 - 66.5%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252 SpA Life Orb Raichu-Alola Thunderbolt vs. 252 HP / 252+ SpD Toxapex: 174-205 (57.2 - 67.4%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
And the last thing I want to note, this is with Raichu running a Timid nature, though he can afford to go Modest if Koko support is closely monitored. I think a mon that pairs well with one of the best offensive mons in the game in a manner that yields Synergy is going to perform a bit better than being just niche.