Personally I think beyond T450 generation, i.e. over 10 years old systems, you are starting to make pretty severe compromises. T440 generation had really bad Trackpoint setup, and older hardware starts to lose features. Random stuff that T450 has that T400 doesn't
* USB3
* Up to 32 GB of RAM (vs max 8 GB for T400)
* M.2 slot (for SSD), 6 Gb/s SATA (vs 1.5 Gb/s on T400)
* x86-64-v3 (AVX2 etc) and OpenGL 4.6
* Dual-band AC wifi and BT4.0 (optional 4G LTE WWAN)
* DisplayPort with 4k@60Hz output
* Slightly larger screen estate (1600x900 vs 1440x900), with FHD 1080p display option
* Dramatically better battery life
* Backlit keyboard
Many of these are not merely nice to have but also ensure longevity by being compatible with lot of other modern stuff. On the other hand I do believe that T450 generation device might remain viable daily driver for a long while still. From the specs the biggest obvious shortcoming to me is the lack of USB-C, especially USB-C charging. But besides that, it seems pretty usable system.
For reference, I have old X240 that I still occasionally use.
* USB3
* Up to 32 GB of RAM (vs max 8 GB for T400)
* M.2 slot (for SSD), 6 Gb/s SATA (vs 1.5 Gb/s on T400)
* x86-64-v3 (AVX2 etc) and OpenGL 4.6
* Dual-band AC wifi and BT4.0 (optional 4G LTE WWAN)
* DisplayPort with 4k@60Hz output
* Slightly larger screen estate (1600x900 vs 1440x900), with FHD 1080p display option
* Dramatically better battery life
* Backlit keyboard
Many of these are not merely nice to have but also ensure longevity by being compatible with lot of other modern stuff. On the other hand I do believe that T450 generation device might remain viable daily driver for a long while still. From the specs the biggest obvious shortcoming to me is the lack of USB-C, especially USB-C charging. But besides that, it seems pretty usable system.
For reference, I have old X240 that I still occasionally use.