This in the long run would make critical hit marksmen significantly easier to balance, reduce the frustration of those playing and playing against critical hit centric champions, and make the bottom lane far more stable.
First, to be absolutely clear, I am NOT referring to multiplying attack damage, only the damage on an auto attack from a champions attack damage, without on-hit effects. For example, Brawler's glove would multiply an attack's damage by 1.1, rather than a 10% chance of doing two times the damage. To break it down, here are the main reasons why this would potentially be a far better system than what we have now:
# Consistent laning phases
Every other role in the game doesn't have the same issue that marksmen face, that being that a purchase has a chance of not actually being worth the gold, regardless of how you play. For example, a Mage's AP will ALWAYS increase an ability's damage, as dictated by its ability power scaling. 70% of 100 AP = 70 damage. It doesn't make much sense to me that someone playing a role well should find themselves spending gold, but not feeling any significant benefit. It's not just about how it feels to play, but also how it feels to go against. To have a higher critical chance than an opponent but to lose due to them landing more critical hits can feel terrible. Instigating this change would simply put, make early purchases such as Zeal more consistent. Players with the item will have a better idea of their champions limits at that stage of the game, and likewise allow the opposing team keep this in mind. A support that's behind doesn't have to worry that there's a chance of taking two criticial hits in a row when going in for a targon or spellthief proc.
Note, this wouldn't also make a winning lane immediately snowball. For example, let's say some point during a so far equal laning phase, an ADC (ADC #1) gets 3 kills after a dive gone wrong, #1 comes back to lane with a storm razor and a Zeal, while the other ADC (#2) comes back with a storm razor only.
Let's say both ADCs have 80 base AD.
1 has 80 base AD plus the SR 60 AD x 1.15 from Zeal, that's a total of 161, against the 140 of #2. This seems a lot for just auto attacks in terms of all in and push power, but remember that a) mages and AD casters such as assassins tend to scale of multiple abilities, giving their all-ins more of an advantage, and b) if this does indeed give too much of an advantage, it can be balanced far more easily. This leads to my next point:
#Overall balance and state of the game
The changes to marksman items in the past two seasons have been controversial, to say the least. The tank mid-season update of season 7 saw critical hit ADCs come online faster than ever before, while patch 8.11 has made them struggle to be effective before 3 items unless snowballed. Switching to the system I've discussed would reduce the likelihood of these highs and lows significantly. Something very important to note is that 2x doesn't have to be the cap. It can be higher or lower depending on appropriate itemisation changes. Pros in early season 8 started to forgo a lifesteal, cleanse, or GA item to build 100% critical hit chance (IE, ER, and two 30% crit items) just because of the high, consistent damage provided. If it's too much or too little, it can be changed with more confidence. Chance should have minimal impact when it comes to a game's balance, let alone 20% of it. Compared to the game's current, and previous state, marksmen would be a lot healthier and allowed to be so.
# Variety in the bottom lane
I'd like to preface this last point by referring to an opinion mentioned by Captain Gameplay where he notes that "you can't outplay an AA". At its core, it's an ability every champion in the game has access to, has 100% AD scaling, and is point and click. Auto attacks cost no mana and they have one of the lowest cooldowns in the game. Adc's almost always need to have the least power in their kit, because otherwise, they'd take over the game. It's no coincidence that ADCs with powerful kits have been nerfed to oblivion (Kalista) or reverted (Kog'maw).
So where am I going with this? The last point I want to make is that if ADC's can scale to guaranteed power later in the game, they could be balanced more around that, thus leading to more variety in the bot lane to capitalise on the early game. We could see some ADC's such as Lucian and Draven be orientated to have stronger laning phases, but fall off after the mid-game, with critical hit marksmen occupying a more late-game role of guaranteed damage at the cost of a riskier early game. Then we can have on-hit marksmen such as Kog'maw and Varus coming online earlier at the mid-game, but struggling against structures. Just as champions that share the same role still have their own niches, marksmen should have that too.
Overall, changing critical hit chance to a guaranteed multiplier just makes sense, to me at least. Yes, it would require a lot of balancing, item reworks, and potentially some champion overhauls to make work, but the long term health of the game would stand to benefit. I'd also like to state RNG isn't inherently bad, but it needs to be handled carefully. I think Dragons are a great example of how to do it right, since players see right from the get-go what type it's going to be, and decide if and how they're going to play around it. A critical hit, on the other hand, isn't apparent until you've been chunked for half your health.
Or maybe I'm just salty. What do the rest of you think?