Assuming that a larger fraction of the world's population is learning 'martial arts', you speculate that the base-level of 'expertise' will go up, perhaps nullifying basic strategies and tactics for self defense. I'm inclined to disagree with this statement for the various reasons. The context of what you mean by 'martial arts' is important. There are, roughly speaking, three facets to 'martial arts' - an aesthetic side, a sporting side, and a more practical, fighting side. If we limit our context to the last option only with heavy emphasis on 'self-defense' aspects, I will attempt to convince you that 'effectiveness' will not change much.
'Self-defense' consists of different levels/layers, with decreasing efficacy as we go down the steps in this list. These are 'situational awareness', de-escalation, altercation, and escape. Without going into the details of each, let's say that you've somehow failed at the first two steps, and are in a sharp, confrontational incident with aggressive tactics. If you've been 'properly' trained (as in striking, grappling, and escape against non-compliant opponents with the possibility of weapons deployment and multiple-opponents), you'll know that a lot of these concepts are optimized only if you get the first strike in - i.e. by surprise. Once your opponent realizes that you intend to retaliate, it's a very different, and complex game, logistically, and legally. It matters less that your opponent may be a black belt in some martial art if you surprise them.
Of course, if they have any serious training and don't let their ego into the picture, most of the time, they will know that a 'street-fight' is very unpredictable and best to be avoided. The most effective martial-art techniques will be simple, quick, and brutal - with a focus on robustness (does it matter if they are bloody, sweaty; or does it use only gross motor movements, do you attack until they are structurally damaged so they can't/won't want to chase you, is your technique sufficiently practiced that you can free up some brain cycles to look of other attackers or deployment of weapons during your attack, etc.). These tactics mentioned here (e.g. surprise first strike, attack until they are structurally damaged, escape) are probably 'future-proofed' in the sense that they are independent of their training. A more comprehensive primer that has a lot more references can describe in more depth some of the concepts described here.