I think this is a completely reasonable suggestion. Personally I am too sentimental about my rings and wouldn’t want to switch even if/when we can afford more. So I went with something beautiful that I think will last and is inexpensive (for me).
If you’re interested in finding an inexpensive set of rings that you can wear forever, here are some suggestions.
I know you mentioned that you already looked at etsy, but there are some awesome sets on there particularly if you look at silver or alternate gemstones. I don’t know if you’re willing to do color, but a gemstone could be an affordable way to get a forever ring now.
Also, metal is expensive. I went with a silver engagement ring because I didn’t want to pay up the wazoo for gold. And there are tons of vintage silver pieces that have definitely stood the test of time, so I think silver can make a wonderful forever ring as well.
There are also alternate metals that are inexpensive: titanium, tungsten carbide, etc.
You can also do a combo, wedding/engagement ring. I might just be wearing my engagement ring as my wedding ring because it’s pretty and I don’t know if I want to get another ring (or if it will even be comfortable to wear two)
Also, vintage rings are often much cheaper than modern ones.
Here are some links that might be useful for you:
Vintage (and it’s size 5 which I think you said was your size)
Gemstone and silver (This is a set of 3 rings — 2 wedding, 1 engagement)
Titanium, gemstone, and wedding/engagement combo ($54 for two rings!)
All three of these options are affordable, pretty, and they should last a lifetime if you want them to. On the other hand, you can always replace them if you aren’t too attached to them and you want to get something fancier later in life.