Complex sentence: The Type 14s size, at just 310 ft, prevented them from continuing past the 1970s and continuing the work they had been doing in the ASW role, as it prevented the Type 14s from being modernised with more effective weapons, thus making them obsolete.
Split into simpler sentences: The Type 14s limited size, at just 310 ft, restricted them from continuing past the 1970s and continuing the work they had been doing in the ASW role. Their small hull limited the extent of modifications and upgrades possible, preventing the Type 14s from being modernised with more effective weapons, effectively rendering them obsolete.