You can see it like that, but you'd be objectively wrong. The effects of a block are not specific to those you've blocked.
Replace 'gankers' in your post with any marginalized or persecuted group and you'll have a pretty good idea how ridiculous I think your logical fallacies are.
'It's always fun listening to the people who defend gingers. Almost obvious most them are gingers, and just too cowardly to admit it.'
Hostile CMDRs, while fairly uncommon, do exist. Preemptively blocking everyone is not always practical, nor can it be reliably done without damaging instancing with people one may be cooperating with that don't fit in one's wing (and if one is only interested in interacting with one's wing, there will be fewer distractions if one takes that wing to PG). Being prepared to fight off or deter potentially hostile interlopers in such a setting is prudent.
As someone who has done a whole lot of this (I'll spare this thread the video spam of my CMDR waking away from overwhelming odds time and time again), I think it's subjectively much more entertaining to encounter almost any hostile CMDR, no matter what my CMDR is doing or intent on doing, than to engage with any of the Thargoid related content I've encountered (I haven't blown up any Titans yet, but I've dabbled in most of the rest). Hell, a large fraction of the game's Thargoid content for those not actively hunting Thargoid's--i.e. being hyperdicted and just having to leave, because one isn't flying an AX loadout--is exactly the sort of encounter you describe, except there is essentially no prospect of variety or any meaningful engagement. Unlike hostile CMDRs, Thargoids have no persistence, have inscrutable motives, and cannot be communicated with. Conversely, half the people my CMDR has met and later engages with cooperatively are those who shot at him first (I'll usually have escaped, or less commonly, shot them down in self-defense, so, unless they were positively incompetent or rudely antagonistic in an out-of-character manner, there will be a degree of mutual respect from which to build a relationship on).
Open, in highly trafficed areas, is not conducive to razor-focused gameplay where interference would degrade one's experience.
I'm clicking Open because the interference, or at least the potential for it, is my gameplay. Even if I've got something specific planned, surprises, even unanticipated obstacles, are not unwelcome...assuming they can be even vaguely contextual.