I use WinMerge[1] a lot, and it's always impressed me how it immediately opens to a useable state. So it's absolutely still possible to write Windows software that can open instantly. I think the biggest issue, which multiple other comments have identified, is that people just don't care. Apps open fast enough these days, and no one is pushing back on developers to improve their app's startup performance.
[1]: https://winmerge.org