The dds guys started getting into 50:0 in the first place mainly as a counter attack vs someone going for the saddle; if you pull the saddle man's lower leg up you can basically slide right into it. And if both of you are already playing footsies, an attacking position is at least better than nothing at all.
It kept working for a while yet since attacks from that position were more or less unheard of and unpracticed by anybody (not for no reason, i would say), themselves included, and also since it's a quick and easy position to get into to begin with (which, in itself, i would say is also a function of it's drawbacks, which we see hence).
Slx/ashi garami gives strong positional control, but for heel hooks specifically it honestly does not have a good angle unless you have a leg knot too (using it to reverse position with an ankle pick/stand up sweep would really be better; or as a point of transition to saddle using a reverse x slide). Getting a good angle to do a heel hook in slx basically means turning your body into a position that is practically like outside ashi/50:0 anyways, losing the control the position has for stopping the opponent from reaching you. Outside ashi is a good angle for heel hooks, but has bad control over the opponent's movement; rather, it can let them control you. This is generally the position you see, whenever you see people 'open to getting punched in the face'; you're basically putting yourself in a hip lock leg turk exposing the back angle.
If for whatever reason the yakuza is holding a gun to your family and you have to do a leg lock from outside ashi or else, i'd say the ankle lock would actually be better there than the heel hook, since the ankle lock itself serves as a point of control, and you can use it to turn (them) belly down.
In general i would say 50:50 and saddle are the best terminuses for attacking leg locks in general in mma (due to the control over the opponent they give, preventing them from hitting you), and attacking heel hooks specifically. While, capacity at using single leg x/x-guard variations in general is also important, even if only as a staging point of transition, since you can slide right into it from a single leg takedown set up.