What happened to hardtails in the past 15 years? So, last time I bought myself a new bike was around 2004 - that was a Trek 4300 hardtail which was a mid-range model and I got it for about $300. I upgraded it with better derailleurs, disc brakes, wheels, better front fork and it had served me faithfully over thousands of miles of road trips for 7 or so years until I'd moved. 

When I moved I picked up a used newer gen (2009 or so) Trek 4500 for $300, had done similar upgrades and had been pretty happy with it. 

I consider myself a fairly typical MTB rider. I don't compete, I don't focus on a single discipline (like BMX or downhill), I would just leave my home with friends or kids on a sunny weekend day and do anywhere from 20 to 60 miles over mixed terrain - some asphalt, some forest roads, some hills, some trails, occasional downhill segment. I'm happy with the workout I'm getting and I'm happy with 26" wheels with semi-slick tires (slick center, grippy edges) which allow me to enjoy riding fast on paved roads and get a decent grip on muddy/gravel trails. 

However, every time I step into a bike shop now, I raise my eyebrows a bit - two things surprise me greatly:


It seems that you can't get a new cross-country bike from a decent brand for less than $2,000 anymore. 
All MTBs have single chainring now and 10 or 11 sprocket cassette in the back. 


On the account of chainring - I am surprised that people settle for much lower max gear ratios in riding scenarios like mine. I read stuff like this and the authors admit that single chainring means compromising your top speed but make the argument that you shouldn't really bother. I'm not quite buying the argument - I find myself using the 3-10 combination at ~90rpm pedaling cadence every once in a while when I want to ride fast on a paved road.

The prices just surprise me - there are no more $300 MTB hardtails in bike shops (or the ones that are there are kids models or horrible no-name brands or knockoffs). I understand that a lot happened since 2004, but going from $300 to $2,000 still seems a bit steep. E.g. browsing through Trek website, I can see that Procaliber 6 is roughly similar in terms of component quality to what 4300 used to be 15 years ago, but 6x price increase seems a bit unjustified. Also, looking at the single chainring on that one - I just don't see why I would replace my good old 4500 with that. 

I'm sure I'm missing something here. What was the generational shift that happened? Are people enjoying mountain biking differently these days? If I embrace the new way, dump my old 4500 in favor of Procaliber, will I experience 6x the fun? What's the deal with single chainring? Is it that much better? 

To focus discussion better - let me ask just one question: 

Why should I buy a new bike these days (other than the "smell of new leather")?
To paraphrase the question: Why aren't there any quality hard tail mountain bikes for $300?

Wouldn't it be reasonable to expect prices to go up since 2009?
None of the following examples are an endorsement. Only an attempt to point out that there are still less expensive good bikes available.

In an aluminum frame Trek has the Marlin 5 that sells for a little over $500.
It does have 29 inch tires but it does have triple chainrings and a seven speed cassette. 

If steel is OK then the Trek 820 is $400.
It has 26 inch wheels and triple chainrings with a seven speed cassette.

Maybe the shop you visited didn't have these bikes on the floor.
Less expensive name brand hard tails with triple chainrings do exist.

Most major brand bicycle companies define a type of bike and a price range they want to compete in. For example "road bikes in the $500 price range" would be a product slot in their lineup. In this case "mountain bikes in the $400 to $500 price range" would be a product slot. Specialized slots their less expensive mountain bikes between $600 and $750. All that to say - Trek isn't the only one with bikes in this slot.

The single chainring bikes you are seeing are the result of the increase in the number of speeds on cassettes and/or a focus on cross country or down hill riding. With as many as 12 speeds on the cassette some people have seen an opportunity to get rid of the front derailleur and simplify the bike for some types of riding.

Edit:
The question shifted while I was writing my answer!
I think this answer is still relevant.