Bit of info for those curious and why this was probably never going to happen.
Radio waves do not travel well through water, like, at all. You need to get down to single hz for it to work, known as VLF, very low frequency. AM radio barely reaches into ultra low frequency, even low hz AM radio (500hz) only penetrates water about 100 feet or so. This means communication is hard. It also means GPS doesn't work underwater. We do have single hz radio systems, Cameron Crow used it when exploring the Titanic and his Challenger Deep sub to the bottom of the Marianas Trench so you can get it and use it but it's expensive so these guys just couldn't be bothered because, you know, jank. Once they got more than 30 feet down they had almost no comms.
When diving, you know where something is underwater because of side scan sonar (like a big fish finder), we aim a sonar sideways and we can pinpoint an object underwater. This gives you a precise location relative to the surface and you can then park your ship right above it. However, as soon as you breach the surface and dive you start to drift and you have no longer have GPS to guide you down to that object, in shallow water you drop a guide line and divers can follow it down but that too can drift so divers/ROV/subs can spend quite a while looking for the target. Getting back to this sub, they sank 12k feet, that can translate to being miles off course by the time it reaches bottom and worse still, the sub is carbon fiber (will that even show up?) and it's wreckage could be mixed in with rocks or Titanic's debris. Basically it gives you potential targets, nothing precise and you have to check them all one at a time, between drift and lack of gps you could spend hours running around looking for the target only to find it was a rock or Titanic debris. People spend years looking for shipwrecks even in shallow water with plenty of natural light. At the depths we're talking here, your field of view is 20 maybe 30 feet. Not only could you keep searching the same area and over due to poor navigation, you could pass by it by 40 feet 100 times and not see it. All that time, just to eliminate a target.
None of the rescue equipment is common or easily transported and none of it was on site.
It's big, it's heavy (it needs to withstand 6000 psi, imagine 2 cars balanced your thumbnail), it needs additional support equipment and people (lots of people). Most are setup on semi-dedicated boats, an ROV and sub capable of this needs a pretty large boat as well as a crane to lift them in and out of the water. It can be moved from one boat to another but it typically takes 24-48 hours to teardown and setup. This equipment reached the docks in Canada last night, it then still needed to travel over 200 miles by boat. It takes a while to arrange, shut down, prep, crate, and move as this stuff is often in use on other projects around the world. You also need a boat with a winch capable of lifting the subs weight plus the 2.5 miles of cable, at that length, a lot of ropes and cables and such can't even support their own weight, let alone the sub, that too requires a dedicated ship. Some ships like the sonar, crane and host ships are more common and nearby ones simply sailed there on their own to await the rest of the equipment and people but the ROV and sub were too far away and needed to be flown in, being so large and heavy the Canadian and US military had to be called in to transport them as most other means of air freight couldn't or couldn't do it fast enough.
The logistics of this "rescue" was crazy and quite expensive and even if you had found it intact it's too deep to use a crew transfer tube, drilling into it to give more air could cause it to implode and the sub has no attachment point where you can hook onto it easily. Even pulling on the strut could cause structural issues to worsen. Which as it turns out this was a problem.
Given all that, and the time frame... Was a rescue really realistic?
That whole "4 days of air" was for a surface rescue, it was never going to save them from anything on the bottom.
Here's what I find the most annoying thing about all of this.
These people spent $250-$350k to ride on this thing and supposedly the company wasn't making a profit. Okay, but money obviously isn't an issue for any of these these people, they were cheap by choice, not because they were on a budget. Can you honestly tell me that if they had charged an extra $40k, $70k, or even $200k more that they would have lost any more customers than they lost due to refunds and people bailing out as soon as they saw this thing? With sonar, gps and comms, they would have known as soon as they were in trouble, if they were alive, determined drift and guided them to a spot were they could easily be located. They could have even had telemetry feedback as well so they would know if it had imploded. You could even install a loop/hook on top (I mean, come on!) and have an ROV/crane combo on standby to rescue as well. Granted, it's very likely none of that would have helped them in this specific scenario but it could have on others and certainly would have changed how this rescue was mounted because we would have known a lot more right away.
