The Mac lineup from 2015 thru 2019 was one of the least compelling set of computers to buy. Fragile keyboards that broke in a year of normal use, high-end laptops with i9s that overheated immediately under any level of use, an annoying and gimmicky touchbar, and so on. If I wanted to absolutely murder a platform, I'd just do what Apple was doing to the Mac in this era, forever.
The iPad division was advertising their product with taglines like "what's a computer" and phrases like "desktop-class" that indicated Apple - or at least, the iPad division at Apple - considered the iPad to be a MacBook replacement, not just a companion. And they still do this, even now when the Mac team is actually putting out good hardware again (ironically, by putting iPad chips in them).
Well... the way the Mac lineup was broken in the date range you have given was less to do with wanting to kill the line, more to do with poor management of the designs being used. It was a very egotistical "we can make a better keyboard", "you want a fance second screen", "we decided for you you don't need ports", "our design is style over substance" type of deal. Whether that was Ive or not, after he left it seems like it all got redone.
You second paragraph - you need to understand the audience. For most average people, they don't need a Mac. They need a web browser, a banking app and a few other sundry apps for casual tasks. For the pro artist, they want a canvas. The iPad fits that fine. With a keyboard it is a very capable computer, with a pen it is a very capable canvas. Even for professional workers, the iPad can be enough if all they are doing is creating documents[1]
The MacBook Pro/Air is for anyone who - want to code in general and/or wants to create apps for iOS and macOS, wants to use addition hardware that requires extra power to be used (music production interfaces for example, or other types of compute module), needs to multitask, needs to consume files from different sources (local, usb, network etc)... etc. So, professional people, or "power users". This includes those that simply prefer a traditional computer. This is a fuzzy cross over. It is not precise.
The M series chips are not "ipad" chips. They are just chips. They happen to be used in multiple places, but this is no different to using an Intel chip in a tablet (which for sure happened. I owned a Windows 8 tablet that had the form factor of a Nexus 7 tablet.)
[1] Anecdote - a guy came to my house last week to do a survey for some heating system changes, and all he had was 2 ipads, a printer and his regular tools. All the documentation and quotes he printed on-site, they all came from the iPad Pro he had with a keyboard. The other one was most used to survey, he had a thermal camera to look for where out underfloor heating was laid as we don;t have the original owners installation documents.
The iPad division was advertising their product with taglines like "what's a computer" and phrases like "desktop-class" that indicated Apple - or at least, the iPad division at Apple - considered the iPad to be a MacBook replacement, not just a companion. And they still do this, even now when the Mac team is actually putting out good hardware again (ironically, by putting iPad chips in them).