I have a new 2019 Giant Roam 3 with the 3x8 drive train. I have osteoarthritis in my left thumb and find the left shifter difficult to manipulate. There’s a shifter upgrade in the future. In the meantime am I wearing my components prematurely by staying on the center chain ring and using the cassette across all 8 gears?

No, and even if you are, they're components (chain and cassette) that need to be replaced on a fairly regular basis anyway. At most, you're probably taking a couple hundred miles off of their lifespan. Being in the center, there just isn't that much crossing at the extremes.

If it comes down to a choice between wearing out a chain or wearing out your thumb, I think you know which one wins.

I love the new 2019 Giant Roam <3

The Shimano Ultegra and Dura-Ace chins cross chain much better than in the past. I've gone to mostly 1X for bikes here.

I don't see any problems staying on the middle chainring except the front derailleur may get a bit sticky to operate the first couple times when you start using it again. Nothing a little use and perhaps cleaning/lubing won't cure.

Many bikes have a single chainring that will be very close to the middle of 3 with derailleur type drivetrains in regards to cross chaining. You may not need the largest and smallest cassette sprockets much anyway.

Its mostly using the big - big and small - small sprocket combos that might be a problem long term. But you can safely use them too on most bikes in a pinch. Just don't make it a habit of it.

I left my bike in the small - small combo inadvertently last week and when I stopped and looked at the chain and rear derailleur I thought it had malfunctioned. The chain was much looser then normal and the rear derailleur looked bad taking up all that chain. Which makes sense now that I think about it. But it still worked fine.

No. There are 1x drivetrains and then there are 1x1 drivetrains.

Don't worry. Use until the gears skip and replace all at one go. Which would be a long time.

Thanks for the input. I’ve done 99% of my riding on this bike with the center ring and will continue to do so without concern.

You really can't cross chain from the center chainring.

Would a twist grip shift on the left work with your osteoarthritis? (~$9 at performance bike online) You could probably continue using the original brake lever, but if not, simple brake levers are also around $9-10 online. See what your local LBS would charge to install.

A twist shift will probably be added the next time it goes to the shop. I had one on my last bike.

Converting to a 1x11 seems like a good option. Shifting would be all on the right.

Your fine.
FYI chainrings are first to go (and are a "consumable" anyway) if you do a lot of cross-chaining and you will be surprised at how many miles they will last.
The rear cassette not to much wear I have found.

I converted my Giant Defy to a 1X11 setup and never looked back. I'm not sure why, but narrow/wide 1X chainring seems to be holding up very well.....better that my old 2x setup (I never used the small ring).

A non-issue, even for people who use all three chainrings. It's never happened to me in the 21 years that I've been riding triple-chainring cranks.

My short answer is that using the middle ring only isn't any worse than riding a 3 X 8 bike. Some people seem to talk like, if you cross chain even once, your bike is going to blow up and you are going die. The reality is that, if you cross chain A LOT, your chain won't last quite as long as it would do otherwise.

My other thought is what's the alternative? If you have a physical problem with your hand, your choices are to either do what you are doing or to not ride at all. If you wear out your components a tiny bit sooner than other riders, that's just part of the cost of riding with your malady.

Yee Gods, can't anyone answer your question? You have no problem cross chaining on the newer bikes. It does cause the cassette and chain to wear a bit faster and you have to correct the front derailleur position but the motions are minimal and shouldn't bother your wrist much.