Only last week I was on a dive in a place known for strong currents where we dive regularly. This time we're hit by an unexpected mother of a flow. We'd already had 1 down current and a couple of up currents. I didn't need a computer to tell me I had an elevated SAC rate. I knew. One look at my contents and dive time told me my consumption was 30% above where it would normally be. What it did tell me was my remaining air time (or rather it confirmed what I instinctively knew based upon experience) and allowed me to make a judgement whether to surface then or get into a better position.
This is precisely the problem with depending on ATR and SAC information. I will assume that your computer calculates SAC based on gas consumption since the dive started and the average depth. ATR is then momentarily calculated based on depth and SAC. I have had tanks heated in the sun to a high temp e.g. high pressure. As soon as we hit the cool water the pressure drops a couple of hundred PSI or more. Our SAC is always high on these dives, which may be good in your situation. Less drastic but the opposite can also happen. But now lets see the issue, you are bouncing up and down in the currents. Your SAC is constantly changing. Your depth may be constantly changing. The amount of effort to get to your start point may have been easier at the outbound and will be harder inbound. And more. Assuming SAC is an average, it will respond slower to current conditions. You might have had great SAC at the beginning of the dive and then terrible SAC in the current. The SAC being used in the calculations will be lower then what it is now. Could also be the opposite here too. Using ATR might return you to the starting point with excess gas (good) or not enough gas (bad). Because you cannot determine the overall situation until back at the starting point, you cannot determine which it will be.
A better method for a dive, especially challenging ones is to use the rule of thirds. Using 1/3 outbound, you call the dive at 2000 PSI (3000 tank), and have 2/3 to get back. I did not need ATR and in fact, depending on ATR may have caused a gas outage. As you stated, you used a lot of instinct here which also indicates why ATR is not a panacea solution.
In the recreational world, I am all for AI but it does not replace using basic principals of diving. SAC and as a consequence ATR is a tool. It is subject to large errors and miscalculations.
In the technical world, SAC is used for planning. One a dive is started, a turn pressure, often a mid point pressure, max time and max depth are used. The turn pressure can be based on rule of thirds or another principal but it is set to know when to end the dive. A higher SAC rate will end the dive sooner. A lower SAC rate will do nothing to the dive as it will end from either the turn pressure or max time.
The information from AI is great but it has limited value. I actually do recommend AI for new divers, including my daughter's boyfriend. To the OP question, eventually AI will replace SPG in recreational diving. Cost is still high, complexity still high, potential issues are still high. Like a lot of 'fancy' BCDs, it is a limited benefit addition but it will be more common. The bigger issue is that I do not believe wireless AI has reached a level of dependability where the 'average' diver can work with it when it has problems. I too often see divers struggling with how to configure their computer. I would hate to see these same divers figuring out why their wireless AI is not syncing. I see many divers afraid to open battery compartments. These will be the same divers that wait for a battery failure to get the dive shop to replace it. On and on. Divers like NetDoc will know their gear inside and out. They will have few issues. Divers that are not 'technogeeks' will need help or more when their gear goes south. Also, wrist mounted computers are liable to be lost or forgotten on a boat. I have seen a Predator almost go over the side. I myself have forgotten to put a wrist mounted BT on and left it tied to my gear for the dive. A SPG console needs service when the regulator needs service. Any other issues are common to diving gear in general, including hose rupture. A good service technician will verify hose condition/age and replace the airspool during a service period. That is about the amount of work the SPG takes. AI takes more service with use or storage as you must replace batteries, remove batteries to protect the electronics etc.
As for the knife - It is a piece of safety gear. Like a SMB or whistle, you may never need it, but when you need it, you NEED it. In 1000+ dives, I have used my knife a couple of times even so, I now carry 2 - 3 knives with me. The SPG/AI is not safety gear. It gives required information throughout the dive, specifically the current pressure in your tank. Any other information is irrelevant to the dive. If you need to know your ATR then you are not a safe diver and you are using ATR as a crutch. That information should only be secondary to your dive.
