So what will the I2C or SPI connect to? Some DAQ system which doesn't have analog channels?
You need to make sure whatever speaks I2C or SPI, supports the ADC you are going to use. With a microcontroller, you can of course create any I2C/SPI protocol you need.
Generally, dedicated ICs are more expensive than the integrated or application-specific solutions when the integrated solution is suitable to do most of the jobs. This is simply due to economics of scale; everybody buys the integrated solutions and only few need the more basic building blocks, making them more expensive. Similarly, building a car out of car parts ends up pretty expensive.
I remember being amazed at the difference of a 16-bit tens of MSPS ADC costing just a few $ when it also included CCD readout control logic, when the bare ADC with similar analog performance but without the CCD control logic costed like $100. Same can be seen in battery management IC's, one with many ADC channels, level shifters, current sense amplifiers is cheaper than any of those subparts bought separately; simply because battery management ICs sell in gazillions.