You know who else has infinite scale passives?
In fact, given that Senna in the lore is closely related to Thresh (not related in the sense "family" but in that their lores overlap and intertwine at various points), it makes sense Senna would have infinite scaling.
Keep in mind that while it's technically "infinite", there is a limitation. At most, it can trigger every (I think) 4 seconds on the same champion - so even if we assume frame perfect inputs of an ability and an auto attack so they hit at exactly the same time, and we assume infinite mana, it could only proc and scale 15 times per minute off champions. And in reality, you're more likely to see a few times (4-5 at most) per minute off champions, and even then it requires quite perfect gameplay.
The ultimate ability is hardly impossible to dodge. The central beam that does damage is 320 units wide. This is slightly wider than Jinx ultimate (at 225), and a decent amount wider than Ashe's ultimate (at 125). So sure, there's two things smaller than it. But then you also have Ezreal (same width as Senna), so it's hardly like Riot's making some massive gamealtering change. If you imagine Senna on the enemy team as an Ezreal, then it's literally exactly the same.
If you include some abilities that aren't necessarily global but still have massive ranges (ex: Kled ultimate, Ziggs ultimate [entire circle], Pantheon [700 units!]).... then you begin to realize this is nowhere near the "worst" or the first.
If you find it easy to aim something perfectly each time by instinct when it has a range of 320 units... then heck, maybe you should be looking into joining a professional team.
