Is rust inside the tubes of a 30 year old steel bike always a bad situation?

I have a mid-80s steel frame (Sannino) that I've had for a couple of years. I recently changed the crank and bottom bracket so while it was open, I was poking around with my fingers and shining a flashlight in there to figure out if it was Columbus SL or SLX just out of curiosity*

I live in a dry climate and never ride this bike in the rain unless I get caught out in it (which rarely happens because I truly hate riding in the rain and will be careful to avoid it). The bike lives like a bat hanging from the ceiling of my garage when not in use. The garage is cool and dry.

The rust is a good layer of dry stuff, but doesn't appear too deep. For example, it doesn't obscure the fairly subtle SLX tube's rifling. The frame itself is in very good condition - original paint and chrome in good shape with no surface rust to speak of, no bubbling, and just some nicks in the paint from normal use which I've touched up with Testor's paint, invisible from 4 feet away.

Any advice whether I should treat the inside of the frame or just be content that it's in good shape for a 30 year old bike and will likely last another 30 in a dry climate

*re: the tubing. An old catalog I saw online said SL so that's what I thought it was. My bike doesn't have original decals and I'm trying to get some good reproductions made, and wanted to get the correct Columbus decals. The frame pictured in the catalog wasn't exactly like mine (painted fork instead of chrome), but whatever. I recently saw a bike for sale online that was identical to mine (except larger) - had the same chrome parts- and it had original decals, one of which was an SLX sticker.

In checking my frame, it's clear that the seat tube has the SLX rifling.The down tube is harder to ascertain because of the way the tubes come together in the BB cluster, I really can't get a good look - it's obscured by the ST. The chain stays have some sort of longitudinal ridges at the BB junction, but I'm not sure if that's a diagnostic feature. I don't know if I'll bother removing the fork to check the DT at the HT, because I think the rifling of the ST at the BB makes the diagnosis SLX. The top of the ST where the seat post comes in does not have the rifling, but that is typical of SLX. From what I've read, it probably doesn't really matter, both are great tubes, with the SLX maybe a little heavier, but a little stiffer (which doesn't matter to me because I'm old and weak). I'd just like to have authentic decals.

I got some good photos of that SLX bike and was able to get good decals made locally to replace the ones I had gotten from an online source that really weren't close at all. I also got some of the correct Columbus decals based on what that are on that bike.