A new pressure release value is like 20 bucks, 40 bucks if you count the big pipe wrench you need to replace it.
10 years is not super old for a water tank depending on the environment it lives in. Its one of those rules like 3000 mile oil changes that does not really apply anymore. I think I have about 15 water tanks over the years, every one of them was old, but they just kept on ticking, so I never replaced them.
Now to the question is a Tankless hot water heater worth the money?
No, it is not.
1. Cost, the unit itself costs 2-3x
2. Install, costs probably 10x more? Tankless uses different venting and gas requirements that all need to be installed brand new
A regular water heater uses 40k BTUs a tankless uses about 200k BTUs. Your tankless is probably going to put your entire gas system over capacity, and there is the rub.
3. New gas meter - odds are you need this, hell you might even need new gas lines run from the street. My gas company offered me a larger one for the low price of $800 bucks (for something that should be free IMO)
4. Usage - Yes you will heat 1 gallon of water more efficiently with a tankless, however if you are like me that is just going to mean a longer shower anyways, so it offsets any savings.
The logistics of this install are a HUGE pita, you involves many trades (gas, electric, plumbing, etc) So that means you get to have lots of different guys as well as a more complicated permit.
Electric is a little easier (no venting) but the issues are the same, you probably need a bigger electric panel, and its less efficient to savings are even lower.
That being said, they are kind of awesome to have!
YMMV