It's actually a damn good, durable engine and arguably one of the best sounding V8s on the planet. The two-piece plugs on the earlier (to 11/2007) engines were the only real Achilles heel, and most of those have already been replaced. The stock powdered metal I-beam rods are strong and light enough for a 7500rpm HCI N/A engine, but they were never intended to cope with the elevated cylinder pressures of FI. In a boosted engine, 450rwtq and 6500rpm seem to be the limit but the rods can bend/break at lower power levels if there's detonation. Nothing that a set of forged rods/pistons plus billet oil pump gears couldn't fix.
The only reason why the 3V doesn't get the love it deserves is because it's been overshadowed by the Coyote. In that case, a Coyote-swapped '05-'09 S197 could be the hot ticket.
I kinda have to agree w/ tjm... in stock form the 4.6 is a little meh. All you need is a custom tune to wake it up though. I never understood why Ford gave it such a sluggish throttle response from the factory.
I don't know if the S197's are ever going to be in as high esteem as a 60's mustangs (too many of them, they're purposely retro) but
relatively speaking I think the 4.6 cars are going to be more like the '64 1/2 - 66 mustangs vs the 5.0's being more like the 67-69. The early ones are still desirable but more people want the later ones thanks to the bigger engine options, more aggressive styling, etc.
As far as dated styling goes...I don't know how much of it is the styling looking dated vs the cars just starting to look old now (Plastic's fading, paint's dulling,etc). The wheels and ride height are a big part of it I think, standard 16/17" rims with fat tires vs what you see now is the same sort of thing you had with the standard 14/15" wheels were on 90's cars when the '05 was new. It wouldn't take too much to clean a car up, drop it an inch or two, and put some new wheels on it and it'd be a lot more contemporary.
I mean look at this (looks like some old used car)
Vs this (could be brand new)
Dated isn't necessarily a bad thing though, it's just waiting for its time. Like people were saying with the fox bodies, it all comes back around. You're seeing the same thing happen with 90's cars since 90's kids have a lot of buying power now and design has moved on so much from that era that it's like a time capsule. Once the 00's kids get into their 30's you could start seeing these getting valuable.
On the electric car subject... I don't think its going to be that bad. I've driven a buddy's Model 3 (dual motor but not the performance one) and the torque is addicting. Isn't that why most of us love the V8? (yeah besides the sound and the rumble)