2010 Mazda5 100K miles: Belts? Tensioner? Idler?

Our 2010 Mazda5 Sport (original owners) recently hit 100K miles. I was all ready to pull the trigger on new belts and pulleys from RockAuto, when I figured I would at least pop the hood and take a look at the original belts. To me, they look fine. There are three marks on the tensioner, the indicator is inside of them. Half of me says, dude, just replace the belts and tensioner and idler and drive it for another 100K mile. The other half says dude, that belt is fine. The manual says inspect the belt every 40K kilometers. Manual doesn't say anything about replacement. Thoughts?
Epping, NH
2012 Mazda 5
I would at least do the belt. I did belt only at 125,000 miles and it had a ton of cracks. Look at the underside where the ridges are.
Thanks @dougsey. I stumbled across this video, which I found helpful. I ended up ordering the two belts and a new tensioner pulley but not the tensioner. Seems like the tensioner is the most expensive part. I skipped the kits because they all included the tensioner. Also I skipped the install tool. This video shows how to do it with a zip tie.
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2010 Mazda 5 Sport
Does Mazda sell the pulley alone? I would prefer that.

ACDELCO is usually good quality, but who knows anymore.

If your pulley is not making noise and still tight...why change it? I don't consider that preventive maintenance because you may be replacing a very good part.
I agree. I couldn't be too choosy with brands because I was ordering a lot of other stuff from Rock Auto and was trying to save on shipping. I had to order based on shipping origin, not brand. I don't know if Mazda sells the pulley or not. I wouldn't be surprised; they sell wiper blade refills. I will try to determine if my original pulley needs to be replaced or not when I change the belt. Since you brought it up I realized I could have checked it easily before I ordered a new one. I have had such bad luck with aftermarket parts, I don't want to use one unless I have to.
2008 Mazda5 GT
If your pulley is not making noise and still tight...why change it? I don't consider that preventive maintenance because you may be replacing a very good part.
This! But this is a wear part and will likely need to be replaced.
One of the belts, the lower I think, is a no adjustment stretch to put on. Do you have one of the tools to put it on?
don’t need a tool to install. All you need are a few zip ties. Cut off old belt, turn pulley, loosely fit belt, zip tie (to guide it onto the track) repeat. Need a tool to take off if you intend to reuse the belt.
I finally got around to installing these parts. Spark plugs were easy, all came gapped just right (1.25―1.35 mm (0.049―0.053 in)). I'm glad I did the belts, they were pretty worn on the inside. The tensioner pulley was a little rattle-y too. I was disappointed in the ACDELCO (1540486 (15-40486)) tensioner pulley I ordered. The original is steel with a name brand sealed bearing. The ACDELCO one is plastic with an unmarked sealed bearing. I am keeping the original. If I need to replace the plastic one I will press a replacement bearing into the original. The tensioner pulley is not hard to take on and off. I should have just popped it off and got the number off the bearing instead of ordering a replacement pulley. Oh well. Live and learn. It took me a few tries to get the serpentine belt re-installed correctly. The A/C belt went on with a single zip tie. I only pinched my finger between the belt and pulley once. Since I was doing the belts and plugs at the same time, I should have taken the old plugs out, then did the belts, then put the new plugs in. The engine would have been easier to turn over without compression. I threw the old serpentine belt in the storage compartment in the back of the car. If the new one ever fails, I can limp home on the old belt.
2010 Mazda 5 Sport
Looked it up...Mazda wants to sell the whole tensioner and not just the pulley so that's kind of out of the question.