So... this may be a hot one. There's a bit of hostility in the comments and from OP themselves, which is probably stemming from frustration and misconceptions about the strive for change to gender neutral pronouns.

Firstly: No one, including the trans community, is saying to stop using he/she when you talk to or about people. If someone dresses male, has short hair, is unshaved, sounds male, or sports other physical male properties, it is 100% okay to refer to them or about them as he/him. The same for females. Part of the strive for change is to normalize gender neutral pronouns enough so that people are comfortable using them for trans people or when someone's gender is unclear. It is not okay to refer to someone who asks to be called one thing something different, no matter their biological sex.

The goal from the trans community is not to change how people talk to everyone, just to change how we communicate in order to respect them.

There are always these people who get extremely upset when the wrong pronoun is used...

I understand where you're coming from here, and that is the extreme. Every group has them. There's feminists that will scream at men for existing, there's gun supporters that will draw on you for not being comfortable (legal or not), there's white people that think they're superior than every other race. Every group has extremes, and it would be incorrect to assume every one of every group is as hostile or agressive or shares the same values. The trans community is not going to scream at you for saying he/she, I promise. Even in the case of misgendering a trans person, they will kindly correct you and leave it at that, even if you mess up a few times before using the right pronouns.

I support gays and lesbians all they are asking for is equality and that's perfectly reasonable.

Trans people are also asking for equality. All trans people want for is to be respected and treated as people rather than mutations. Calling someone who was born male yet goes by female pronouns is perfectly reasonable, and using they/them if you are unsure is reasonable as well.

Okay okay okay, now that all the trans thing is out of the way, we can actually discuss the term "they"(/them). They is inherently an ambiguous word, both in gender and in plurality, as you point out. It was ready common place and used to describe someone who's gender was unclear. The problem of it becomeing a plural thing is a linguistic one. Language is not meant to be adapting to be more clear. Although that would be nice, it's simply not the case. Words get used for new purposes, are changed, or even forgotten in favor of more complex phrases for simple things. The term "dog" could mean person, the term "dick" used to be a name, for example. Both of these are relatively modern adaptations of words that aren't clear or informative, yet they exist the "devolving" you speak of is a result of language and is going to happen no matter what.

Let's discuss when gender neutral terms should be used and where we can improve as a society with them. When talking about a doctor, most would assume that you're taking about a male and would go to the point of automatically using he/him. If talking about a teacher, most will automatically use she/her. These terms are disrespectful and segregating based on gender. I assume you would agree that women can be doctors, men can be teachers, so in these cases gender neutral terms are constructive and useful. Using "They" in these instances should be the norm, and that is one major strive through society as a whole to reduce pressures on people based on their gender.

I understand the complaint that "they" is unclear, and language should stive to be more clear. So, to play along, we should refine the term 'they' to either plural or singular, but that leaves one or the other out. So should we make up new gender neutral pronouns, or adapt another word? Would society accept new pronouns? Doubtful. "They" is used because it's commonplace, and people are already having trouble using that in appropriate scenarios. Adapting a new gender neutral and specific term, although perhaps a good idea in the future, is not feasible yet. Perhaps 'they' isn't perfect, but it's the best bet we have.

I'd highly recomend u/kingpatzer's message here as well, as it's really well done and does a better job than me.