In which case you can control the charging speed via your choice of cable.

Correct. Unless they've introduced it since my Pixel 2 stopped getting updates in November, Google do not allow you to choose the charging speed or restrict fast charging via the phone's Settings (some phones do, e.g. at least some Sony models, but not all).

Many people have questioned it. But most of the industry doesn't care, and the blogs/reviews have all swallowed the "faster is better" groupthink.
Subjectively and unscientifically my Pixel 2's battery has degraded a bit faster than the HTC One that preceded it. Not catastrophically, it's not died after 1-2 years, but at 3.25 years old the battery degradation (about 72% capacity) is more like the previous phone was at around 4 years.

Do note though that one person's observation is merely an anecdote, and not evidence of a consistent systematic effect.

No. Charging speed is controlled by the phone, and the phone has no such control available to the user. And any PD-compatible charger will be able to deliver more power than that.
I doubt you'll be able to find a dodgy cable that can only deliver 10W, as (a) I expect that if the phone decides the cable isn't up to spec it will charge a lot lower than that, and (b) if it doesn't it's likely to be a fire hazard .

 

Marketing.

These days a phone is slated if it doesn't support fast charging, so there is no choice about that. Even "only" supporting 18W charging is likely to result in it being criticised in reviews for being "slow" or "lagging behind the competition". But they get no criticism for not providing the option to turn it off. And Google aren't renowned for providing lots of options in their software, they are very much a "we know best" company, so it's not hugely surprising that they don't provide this. But bear in mind that many other manufacturers also do not provide such an option - what Google have done is the norm in the industry these days, not the exception.

I admit that when I see headlines saying that phone X supports 45W or 60W charging (or even higher) my response is "I'm not buying that". But that type of thing gets approving writeups from the blogs, so don't be surprised if it becomes more common.

 

Some Samsung phones have (or used to have) and option to disable fast charging.
Recent Sonys have an option to set a limit to how high it charges (so if you prefer to only charge to 80% or 90% you can set that), and also have features that claim to spot regular charging habits and adjust charging to match (e.g. plug in overnight and it will delay charging so that it only completes shortly before you normally get up).

 

I'll confess that for my next phone I'm tempted to just dig out one of my old chargers for the bedroom with an A-to-C cable and just opt out of fast charging. I can see the utility if I need a quick top-up (but I can just plug it into my laptop if I want that), but as the default charging mode it doesn't fit my usage.