It's not like you can't learn the product through the PM either.
If you're a true senior software engineer as most of us claim to be coding is a small part of your job, not your entire job.
You should be learning the product through the PM for sure, and I don't think a senior engineer should be doing first-level support, but especially in small companies talking to customers is good and should be expected from basically everyone who is working on the product.
"the PM can't be expected to sit in meetings all day, they need to learn the coding side of it too so they know the potential limitations of the features they want to suggest"
But if a PM does have a technical question, they don't need to go google stuff and figure it out - they ask a developer.
Likewise, when a developer has a product question, why can't they rely on a PM to answer that for them? Why must we also be expected to be in customer meetings and putting in extra effort, when PMs definitely won't put in effort to learn the technical side?
That's not going to prevent the PM from asking questions to the developers though. I ask questions all the time, because I want to validate my mental model with others and verify my understanding. Asking questions is a /good/ thing.
The part where you are missing the boat is acting like customers are a distraction or an enemy. Customers are /the point/, the /only/ point, really at the end of the day. Every role in every business is customer-facing to some degree.
In a good organization they first try to figure it out themselves versus distracting a developer (thus costing possibly hours of productivity due to breaking someone's flow). The same way a developer would first try to answer their own question before they start badgering another developer.
They've already gotten away with adding infrastructure and architecture (aka system design) rolled into one developer position. And putting it behind long and stressful interview processes. I'm not doing PM stuff on top of all that and not getting the pay and prestige for it.
Perpetually doing less in fear of not getting paid enough for doing more is how you get paid a pittance while complaining about it constantly. Doing more and then finding a way to get paid more is how you get paid more and be happy.
Having engineers handle "support calls" doesn't make much sense, they are not equipped to manage product feedback or understand the business implications.