On a basic build with no other crit chance, just Assassination alone, 40% crit chance is a very low baseline, something that would take multiple 5 piece sets + non-class skills + specific weapons equipped (daggers) just to reach, completely nullifying the ability for a build without Assassination to slot in extra bonuses in other areas such as crit damage, pen, sustain, raw damage, etc.

Note that most competent builds will sit around 5.5k to 6k weapon/spell damage, so stacking to 7k is more like a 15% increase, not a 20% increase in damage done that I am using for these calculations.

Assuming a baseline 100% crit damage done (achievable on a weapon/spell damage build, but will likely only have ~20% crit rate):

Stacking weapon/spell damage will increase the base damage of the skills by about 20% on average, but they would still only have an ~20% crit rate. This means that the crit chance is only effectively providing a 20% damage increase to those skills on average (20% of 100 is 20). Adding the +20% increase from weapon/spell damage buffs, that's approximately a 40% increase in damage total on average.

Stacking crit rate so that it reaches 50% (something beyond extremely easy to do when using Assassination), even though there's no additional damage buffs to those skills from stacked weapon/spell damage, the crit rate alone boosts the average damage done by 50%, which is significantly higher than stacking weapon/spell damage. This also assumes only Assassination and no other sources, it's very easy to reach 60%+ crit chance with Assassination slotted + sets like Null Arca making this difference even higher).

This disparity when stacking crit chance compared to weapon/spell damage is also increased when looking at the meta build itself that easily hits the 125% crit cap (often sitting at 140% and I have seen 160% possible to completely mitigate enemies crit resists, even with rallying cry active).

20% of 125 is 25, so stacking the 20% weapon/spell damage with the 20% crit rate at crit damage cap would be a 45% increase in damage total on average (still below the 50% from stacking crit chance at 100% crit damage), while stacking crit chance increases the average damage done from 50% to 62.5%. Widening that gap from 10% to 17.5% (nearly doubling that gap)
Scale this up to the 160% crit damage (enough to ignore enemies crit resists and assuming this crit damage stat is possible to reach on a weapon/spell damage stacking build), and you're looking at 52% increase to damage done on average for a weapon/spell damage stacking build, meanwhile a crit stacking build reaches a whopping 80% increased damage done on average.

This also applies to healing, where there's significantly less ways to buff crit healing. Even at a reduced 80% bonus to crit healing (easily reached on a crit build, even not that hard to reach on a non-crit build), this still puts it on par with stacking weapon/spell damage in terms of average output.

I don't have the exact math/formula behind weapon/spell damage stacking, but to match the current average damage done increases that crit chance provides, you're looking at probably around 8k weapon/spell damage at the bare minimum and this value is more like 10k+ weapon/spell damage when using crit damage cap values (125%) instead of significantly lower values that would be on a damage stacking build, not a crit stacking build.

To reach such values to match crit rate stackings average increases, you're looking at a full PvE DPS build (with group buffs), which has zero survivability, or slotting storm calling + aedric spear + minor brutality/sorcery (dawns wrath or earthen heart) + 2 5 piece weapon/spell damage sets + multiple named buffs, just to reach the minimum threshold and this build wouldn't even get close to the 100% crit damage done stat I used for my values above without massive group support which would make that minimum threshold even higher than previously stated.

Meanwhile stacking crit chance, just slot Assassination + the current BiS 5 piece set (null arca) to match the same increases to damage that stacking all that weapon/spell damage required above and the rest of the build is free to use whatever else I want to stack up other stats as needed/desired.