Does it make sense to switch to single chainring on an older mountain bike? Very recently I've started seeing a setup like this on pretty regular bikes:



(Image taken from CyclingID forum)

Appeal of the single chainring

To me the positives of this setup became quite obvious the moment I saw it: fewer parts to clean and maintain, no "dead" chainring-cogset combinations (and the occasional "thumb fight" that comes with it), less clutter on already busy handle bar. Weight is not an issue for me (see below). Price for many new parts is the single disadvantage I can think of, but I'm doing it for fun and money isn't a big problem.

My current bike

I have an older (10-15 years) 26" aluminium frame mountain bike with a suspension fork and hard tail. Shifting is Shimano SLX 3 x 9 with short derraileur tension arm and some very worn out levers.  I upgrade and update the bike regularly for both practicality and the fun I get from taking the thing apart and making it a bit nicer. The scale of upgrades goes along the lines of upgrading the supermarket-grade fork to RockShox Tora Air, basic Shimano V brakes to Avid Juicy 5, nice sturdy Remerx wheels and so on. Weight is too horrendous to even mention but I don't mind (and don't want to mind). So basically a very expensive fairly crap bike. The original basic Shimano chainrings are getting old (and too worn, I fear) and I'm thinking about he best replacement.

The question: a small update or big upgrade

The chain, chain rings and cog wheels need replacing, and I'd happily swap the old (and perhaps a bit worn, too) square tapper bearing bracket for something nice as well. I'm not happy with the overal gear change function (caused by old age of the system from levers through wires to improper part combinations I came to over the years) and I'd like to overhaul that as well. So there are two options for me (see further down still), and I want to avoid the possible trap of the appealing and fashionable new thing, the silliness of which I fail to imagine in advance. Are there any downsides of the second option apart from higher part costs? Is there something important I am missing in the shopping list for the second option? Does the second option even make sense?

Option 1 - What I'd upgrade anyway: Chain, cables + cable housing, gear change levers, frame bearing bracket, crank arms with chain rings, cog wheels.

Option 2 - Upgrade to single chain link: Chain, fewer cables and cable housing, one more expensive gear change lever, more expensive crank arms with integrated bearings, new rear derailleur, a new hub with all that goes with it if necessary (I'm afraid it will be).



Update I don't want to buy a new bike even though I'd get more of a modern bike for less money. I'm drawn by the prospect of less shifting and less maintenance and am looking for possible drawbacks or obstacles one can't imagine until one bumps in them mid-upgrade.
The more general form of the questions is: is it worth upgrading a bike vs buying a whole new bike?

In many cases it is not worth it. Individual upgraded components are compromised by remaining lower end ones. Full upgrading of components is as expensive as a new bike. The bike is still saddled with older technology (such as rim brakes).

However, if the point is to mess around wrenching on a bike, give it a new lease of life, the components to be replaced are worn out anyway, and the limitations are well understood; then that in fine. Especially so if the builder is prepared to spend some time on Craigslist and Ebay finding cheap serviceable parts. 

[Full disclosure - I own a mountain bike with components spread over two decades.]