Synthesizers provide a wide array of different types of instrument in one housing (brass, strings, pads) in addition to effects and sequencers. This means a synthesizer can supplant what would have classically been one or many different people in a band. The same goes for drum machines.

DAWs provide most of the benefits of synthesizers and drum machines with the additional benefit of being a production studio.

You rightly point out the barrier to cost is being reduced but, in my opinion, this goes hand in hand with complexity. After all, I "could have just hired a band" to experiment, play around and bring my vision of music to life. Is putting a band in a computer for less than $300 "just a matter of reducing cost"? Encapsulating complexity is often synonymous with reducing cost.

Any tool, even ones that are cheap and/or encapsulate complexity, require training to use well. The question is whether they can be used "professionally" with a modicum of training or whether they need a lifetime. A good artist can probably be proficient enough with a synthesizer, drum machine and/or DAW in a year or two before starting to create quality work. It takes a lifetime to become a professional at the piano or violin, often requiring exponential more work for increasingly marginal gains.