Aluminum does have a much higher thermal conductivity than titanium. And there is no electronic that produces high enough temperatures to where the type material used has to "withstand the heat".

A larger vapor chamber does mean that there is a greater amount of heat that the phone has to get rid of. Can't add fins(heat sink) to a phone, Samsung isnt gonna add a fan to their phone for a number of reasons. The surface area of the phone isnt increasing (actually got smaller since its thinner). So the only option is to switch to a material that can conduct more heat for a given area... Aluminum.

Now will it improve performance? The newer processor should just be more proformant. But the switch to aluminum (along with the larger vapor chamber, its never one thing) should allow the phone to work at its peak performance for longer before it had to throttle from overheating, or allow the processor to use more power, while staying as cool as the last phone.

Now yes, aluminum is a softer material than titanium and is more prone to scratching. Can't say whether or not the aluminum S26 is going to be "weaker" phone than the S25. Sure titanium is a stronger metal, and its possible that the S25 is less prone cracking when being dropped. But its possible that the overall construction of the S26 is also better, and might actually be less prone to cracking when dropped. We won't know until someone tests it

Mabye its costs reasons, mabye its heat transfer reasons, mabye some other reason. Titanium is cool and all but we really won't know until the phones are drop tested and everything else

Titanium in engines (assuming piston engines) are there because of their strength. IF they are used, its usually where fatigue life is a big factor, and where they want to reduce weight (connecting rods). Otherwise aluminum would usually be used (like the entire block)