1. It only can't unilaterally if you expect a major increase in the amount of stake voting for witnesses. That includes stake held by dormant and even dead stakeholders, lost keys, etc. I don't expect this.
2. Even if you assume that there is enough stake actively voting such that 37.5% isn't enough to literally exert unilateral control, it is still highly dominant. With the slightly simplified model that there are two distinct slates of witnesses and it takes >50% to vote in one slate, the 37.5% only requires 20% support from the rest of the stake to reach 50%. This could include app owners with stake who are bribed (which need not be directly nefarious, but could include offers of funding for marketing, development, etc.), people who are confused, people who are swayed by marketing/campaigning, etc. Getting 80% of any group to agree on anything is not easy, and certainly much much harder than peeling off 20%.

But realistically you are not going to get all of the rest of the non-voting stake to participate, so even if you could possibly get enough to participate such that 37.5% is strictly below 50%, it would not be 20% that needs to peel off but maybe a few percent at best.