There are a few reasons why this doesn't work as well. The Striaton trio are teaching you a lesson you've already learnt. At this point in the game, you've fought Cheren, who uses the starter your starter is weak against, and Cheren's starter knows the relevant SE move (Ember/Vine Whip/Water Gun). If you were just using your starter up to this point, you'd have run into the Striaton trio already, which makes them just a repeat lesson - rather defunct.
There's no harm in explicitly teaching a lesson through visual example, rather than relying on the chance of an opponent using a move that is SE against a Pokemon that you might not even be using at the moment.
Lenora isn't a strong enough leader to force you to vary your coverage. Look at Yellow's Misty. She had a level 22 Starmie. That's base 100 Sp.Atk, 115 Spd, probably a good 6 or 7 levels above what you'll be on reaching Cerulean the first time, firing off STAB'd Bubblebeam which has more base power than any move you'll learn for a fair while. Misty is tough as nails without an Oddish/Bellsprout/Pikachu (unless you picked Bulbasaur). Lenora is not. Herdier and Watchog don't have the same threat level as Starmie, Lenora's team is a lower level, etc. The only real advantage Lenora has is STAB'd Retaliate, but it's only coming off base 85 ATK. You can brute force your through Lenora, which is a lot more difficult for Misty.
Gonna have to disagree here...
Misty
lv. 18 Staryu: Tackle/Water Gun
lv. 21 Starmie: Tackle/Water Gun/Harden/BubbleBeam
Lenora
lv. 18 Herdier: Leer/Bite/Take Down/Retaliate
lv. 20 Watchog: Leer/Crunch/Hypnosis/Retaliate
Now, on just levels alone, these teams are basically the same. In terms of moveset variety, there's no arguing here - Lenora's team clearly has more variety, and more dangerous options here, with much wider type coverage.
Using Retaliate as an example, it definitely hurts to have a 140 STAB Normal move coming at you, especially since there aren't many options to tank that hit at that point in the game (whereas in RBY, you could maybe resist BubbleBeam with any Water/Grass types you have). Even without Retaliate, if you don't have something that can immediately take care of Watchog you risk being put to sleep by Hypnosis, or being flinched by Crunch. Starmie is more one-dimensional with just having the weak Tackle as an alternative.
Using *team levels to that point in the game* as a comparison point, both gym leaders have a similar run up, level wise, before their cities. Mt. Moon has the lv. 8-11 range represented, as does Unova Route 3. Evenly trained teams will essentially be at similar levels before these gyms, with RBY holding the advantage in wild grinding due to the lack of experience scaling. There's a greater chance of a team being prepared level-wise for Misty than Lenora. BW in general has added challenge with the anti-grinding effect of exp scaling, forcing you to be a little bit more creative in team makeup.
To an extent. Unova Dex is shit for Fire types, though. Kanto is poor on Dragons and Ghost types, although back then Dragon was the "boss" type given no trainer prior to Lance ever uses a Dragon type and Dratini and Dragonair are extremely well hidden (Super Rod in Safari Zone with a difficult catch rate), so that's an excusable omission. Similarly, Ghost types were related to the Silph Scope.
The problem with actual Pokemon chosen, rather than typing, in the early gens, is not really the Kanto Dex's type variety, which is pretty good considering Kanto has a 150 dex and is somewhat on the smaller side, but more the fact gym leaders just seem to use 5 of the same Pokemon line for no apparent reason. That does bug me.
The Ghost type is one of the bigger problems in G1 because of that whole programming bug, as well as there being no actual attacking Ghost moves save for Lick. Similarly, Bug types were shafted due to being the designated weak type, only good for quick early evolutions and little else, as well as lacking any good moves. For this reason, Sabrina falls out of the whole "hard unless you find x Pokemon" lineup, because going to catch Gastly at Silph Tower is useless, and would put you at a disadvatage, due to all 3 Ghost Pokemon being weak to Psychic due to their typing. Because of this, it can hardly be said that RGBY has the perfect lineup of gym typings...
Speaking of "bosses", it's interesting how Dragon is resistant against the main types (Fire, Water, Grass, and Electric - starters plus Pikachu). However, there's nothing that would lead one to pick up that Dragons are weak against Ice types - I believe this is why Ice Cave was placed before Blackthorn City in GSC.