Would It be fatally dangerous to ride with this dent [Pictures]?

There are bicycles that don't have a top tube: step through frames. Although they have a bit of support usually at the area between the seat tube and top tube, they still approximate a bicycle with no top tube. They are not stiff at all so I suspect a sporty rider would not use the step through frames.

Bicycle frames have lots of inherent redundancy. If a tube fails, it is usually not catastrophical: the other tubes can perfectly well support the weight, and you notice something's wrong with your frame as it feels different than usual, and then notice your tube has failed.

The dent looks pretty bad. If this was on a fork, I would say you are putting your life on danger by using it. Forks have practically no redundancy.

With frames, I'd say you are not putting your life in immediate danger by using it. Still, it's pretty substantial damage so I would avoid riding the bike unnecessarily before having a replacement (order the replacement today not tomorrow!). If you can't work remotely and can only commute by bike, then perhaps you might carefully ride it to work, avoiding any maneuvers that put excessive load on your bike like braking hard with the front brake.

I don’t think it’s bad enough to immediately present a hazard. You should be fine as long as you ride it sparingly (ie not for three hours every day or something), and you avoid all gravel roads, cobbled streets, potholes, curb jumps, etc. Since you know the frame is done for anyways, I’d strip the paint off that area so you can better watch for cracks, further buckling, or any other sign of failure. If you see anything unusual, immediately stop riding (even if that is mid-ride).

I also must ask, how heavy are you? A lighter rider isn’t stressing their frame as much as a heavier one obviously.

My opinion is, in short: buy a second bike.

The long version: if biking is so important to you that you can't wait 8 weeks to have it inspected by someone who is really qualified to answer, having another bike is a good strategy to keep riding if your bike is immobilized. And then why not something from a different category, to avoid having two redundant bikes (otherwise the spare one will just collect dust and might no be ready when needed).

It doesn't have to be something expensive: to give my personal example, the second bike is a trekking bike, that I use for more utilitarian tasks (going errands, when I go to visit friends,...) or riding with my wife — who rides at a more leisury pace. But it can also be a gravel bike or a MTB, it doesn't matter as long as you like riding both of them.

Your frame is probably toast.

Aluminum tubing is much thicker than steel tubing, so it's possible you'd be ok, but I highly doubt it.

Now, I personally would ride it home with that kind of damage, but I would ride pretty slowly and carefully and as directly back as I could, but once I was home, that frame becomes a decoration.

And in any case, if it's not toast, it is for sure the end of the frames life as a racing/performance machine. It could have a life as a beater bike, but it would take a lot of slow riding to build up the confidence it's not going to fail on you. Better off giving up on it.

Your frame is perfectly fine until the instant it isn't.

That instant will be when the frame is under the most stress laterally, which is during high-speed/high energy changes, like turning/braking at speed or an impact with a stone/pothole. At that point the dent becomes a focal point, and will form the apex of a fold.

My gut feeling is that the tube will push to your left, with the existing dent in the "armpit" and you will likely fall. Remember the edges of the fold have suffered a crease, and are now work-hardened and permanently weakened.

Whether this failure comes on your very next ride or in ten years is impossible to say.

The only defence is to examine the dent before and during EVERY ride and if it shows any change or deterioration then expect imminent failure. HOWEVER there is no guarantee that the dent will show any change until it fails.

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Your best and safest option is to scrap the frame, and move all the components over to another frame of similar tech levels. Shop around and see what's available on the local used market, or ask your LBS in case they have anything in stock.

Ideally you want a frame that uses components of a similar age, and will let you move your fork over. OR get a replacement frame that comes with a suitable fork already.

Example - here's a mid-2000s chainset and some 2010 wheels on a 1980s frame:

This frame died due to old age (you can see the paint missing on the seat stay - it was dented there. Most components were moved over to a mid-90's cannondale frame which I purchased retail:

The handlebars and stem and saddle moved over, but I needed a different seat post. Wheels and transmission and crankset moved over along with the BB cartridge, but I needed different brake calipers.

Even the cheap bottle cages moved over, but the mudguards didn't due to lack of clearance.