P2SH was something that everyone was supposed to be using by now because of the "obvious" security need; you're proposed reason for a soft-fork and associated system-wide risk is significantly more niche.
It won't be niche when the PC platform as a whole is rendered unusable for secure applications by firmware-level, airgap-crossing malware.
*Secure hardware wallets need to be here before that happens.
*"Secure" means custom silicon specifically hardened against side channel attacks and such, not off the shelf embedded systems containing god-knows-what vulnerabilities.
Custom silicon is the problem: how do you know what it's actually doing?
The sweet spot is probably 16 and 32-bit microprocessors: fast enough to handle crypto without pain, small enough that hiding malicious features is very tough for the manufacturer, and cheap enough that the community has a chance of auditing the actual shipped hardware and firmware against the claimed design.
Microprocessors in this class don't have significant problems with txin's provided they have a reasonable interface to the PC, like USB: just hash the supporting transactions incrementally. With well-designed hardware the fact that the device is connected by USB has no security risks and can be easily audited. (FTDI's USB<->high-speed-serial chips are a good option for the truly paranoid)