The companies involved have researched the market and the needs of the market and come out with what we have now, trying to argue the end result is wrong would need data to show otherwise.
From an engineering stand point, integrated/monolithic hardware fails less, there's a whole class of failure scenarios removed (ie connectors and interconnects, better sealing, etc).
Hardware has reached the point where it's on the same level with every-day things people throw away and buy new versions of without even thinking, like kettles or toasters, it's fine to argue that's wasteful but that doesn't stop most of the world from doing so for practical reasons.
No, technology companies are researching how to make more money. That frequently means making changes that are not in consumers' interests, in particular when it comes to either closed ecosystems and lock-in or when it comes to built-in obsolescence.
See also: "Smart" TVs, the history of pricing in the printer industry, Anything-as-a-Service, multi-year contracts in the mobile phone industry that obscure the true cost of these devices, DRM, and countless other trends in technology.
From an engineering stand point, integrated/monolithic hardware fails less
In some cases, yes. But when it does fail, you lose the entire device. It is far from clear that any benefit in reliability is a net win relative to any loss from making all failures catastrophic when they do happen.
Hardware has reached the point where it's on the same level with every-day things people throw away and buy new versions of without even thinking, like kettles or toasters
Not even close. That's just what people who make a lot of money selling new devices that don't actually offer much benefit over "old" devices want to convince everyone to believe with their marketing.
The current wasteful attitude is not sustainable, either economically or environmentally. We've only had this "two years and you throw away your $500 iThing and get a new one" kind of mentality quite recently, and most people have been through these kinds of change no more than once or perhaps twice so far.
Just like no-one doing corporate IT assumes you'll throw away a PC every three years any more, the market will learn, and probably very quickly. I already know plenty of people who have had exactly one generation of Apple gear and then become frustrated with the short lifecycle. I know almost no-one going the other way. That's just anecdotal of course, but looking at market share trends and general commentary over time, it looks like the experience and sentiment within my network is not unusual.
That might have been true 10 years ago, when typical speeds and storage capacities were increasing dramatically from one generation to the next and those increases were actually useful.
But even entry-level PCs have been able to run typical home or office applications -- things like reading e-mail, browsing the web, writing a letter or putting together a spreadsheet -- for a very long time. You don't (or at least shouldn't) need an i7 hexacore processor overclocked to 4+GHz and 16GB of RAM to write a letter. The only sharp improvements in recent years have been high-resolution displays (much more important on smaller screens, but generally a noticeable benefit) and SSDs (particularly in much faster boot/load times on otherwise modestly specified machines), and for applications that need them, GPUs.
Even for more demanding applications that really do benefit from greater RAM and processing power, the real world specs for both processors and typical system RAM have barely moved the needle for probably 5 years now, with the emphasis more on low power consumption and more cores within a single CPU. The cost/benefit for upgrading even a high-end workstation from a few years ago to the state of the art today is rarely favourable.
The biggest problems with longevity today, in my experience, are monolithic hardware where components can't be swapped out on failure and, more than anything, software. The reason my third-generation iPad isn't as useful any more has nothing to do with the hardware spec, it's because Apple chose to write more recent versions of iOS in a certain way and then forced the app ecosystem to upgrade because they have absolute control via the App Store. Now we have a choice between not using a lot of newer apps or upgrading to a new version of iOS (which is widely reported not to work reliably on our equipment, and with no way to downgrade again if those reports prove to be accurate in our case). All of this is entirely due to artificial problems caused by a throwaway software culture, and nothing to do with the hardware.