Sound Reinforcement Systems (P.A. is strictly incorrect, as it refers to a paging system, sometimes called a 'Tannoy' after the manufacturer who used to service railway stations) permit the audience to hear. Okay, the standard varies, but that's another thread, perhaps.

An audience, no matter how well-behaved, generates ambient noise. Some rooms are acoustically better than others. Enabling the audience to hear properly is not always within the control of the performer... I prefer to work without a sound system, because sound systems are my day job. However, where necessary, I can and do use a sound system.

Maybe there is a case for limiting the size of an audience to enable a truly acoustic performance, but in reality there is the problem of making it pay. I doubt that a small, select audience would agree to paying a high ticket price... so it's something of a trade-off.

Backing tracks, on the other hand, are not provided to help the audience hear. It would not matter if the audience was two or two thousand! It's simply a matter of etiquette, and what the audience expects.

If a performer is intending to sing in front of an audience they should show respect for what the audience is expecting of them, either by using an instrument played live (with an accompanist playing if they cannot themselves) or putting together a set of unaccompanied material.

I'm sorry, but there are plenty of capable people around, so let's not pander to those who either cannot or will not accept the etiquette of a LIVE performance. Backing tracks could then lead to 'lip sync' (miming), which is not what audiences pay to hear.

By 'pay to hear' I also include people who are paying with their time, not necessarily with cash.

Once a trend has started, it's very difficult to reverse it!