IBM's Future Systems project tried to replace the System/370 world with a radically new architecture and software environment, but migration risk, performance concerns, and too many simultaneous breakthroughs made the corporate-wide rewrite untenable.
Apple's Copland project tried to modernize the classic Mac OS in one giant step, but feature growth, architectural fragmentation, and slipping schedules convinced Apple to abandon the internal replacement and acquire NeXT instead.
Netscape's browser rewrite created a multi-year release gap just as Internet Explorer was gaining distribution power, making Netscape 5 a classic rewrite warning—but not the sole explanation for the browser war's outcome.
Microsoft's Longhorn project accumulated an ambitious new storage system, presentation stack, managed-code experiments, security work, and shell changes until the Windows team reset development onto a stable Windows Server 2003 base.