(fixing things) I think this comparison should solve your dilemma (other than my “fanboy” like talk which is not intended to be like that. Business is serious.)
If a big software does not evolve, it is like you will always fight with a sword instead of improving to guns over time.
So the guns of others (other Engines) will beat your sword you’re desperately trying to hold on during the fight. The community moves over to those with guns to avoid fatal death in business (which is… serious). So the swords become rusty. The Software just dies because its getting old.
This is actually what you are asking for, but you also don’t want to happen 
UDK was a sword competing against other swords, but now guns seem to be better. Still your choice.
Of course you can reuse old libraries and old knowledge with some tweaks. Think about bayonets 
UE4 has remaining swords as well. Just let us be exited about plasma cannons!