1) The only thing that systemd might do faster is boot. Since Linux servers are not booted that often, that is a trifling advantage, at best. Certainly not worth breaking everything that works.
2) Systemd does not always boot faster. Only under certain circumstances.
3) More resource intensive generally means slower on the same hardware. Systemd may boot faster, but it runs slower.
4) There are ways to improve boot speeds without breaking everything that works.