The general answer to this is that the requirements drive the architecture of the system. Specialised components are, generally, more expensive and require interfaces, which increase the integration complexity and therefore cost and time to market. However, your requirements might dictate that you need the performance of the dedicated component. To take a consumer computing example, most people just use the internet and office tasks and so they don't need an expensive GPU, the in built graphics driver on the processor (SoC) is fine. In this way, the motherboard doesn't need the expansion slots, high power supplies, size of case etc.
For your IoT case, if you're just polling a sensor every 100 ms and then sending it out over a BLE link every 5 minutes, you'd chose a SoC that has a low speed ADC and a built-in RF module. The vendor will likely provide example code, so you can be up and running in a day or two. Then, when you need millions of samples per second, but can process that down into something that only needs a small bandwidth, you might be able to use the original SoC and a dedicated ADC IC. Then you decide, actually you need a higher bandwidth output, you move to a dedicated ethernet controller. Each time you add you a new component, you increase the complexity and hence cost by any measure (time, money, size, errors...). So, your concept is valid but always think if it's necessary by whatever metric is valuable to you.
At this point, the system is has 3 different ICs and interfaces, but then you discover that many FPGAs are able to handle this in a single chip, and you move to that instead!