Re: "there are two aspects here – performance and functionality."
Because people quickly found serious technical limitations with bubble memory.
* It couldn't run cold. You had to warm it up first, and keep it warm. Fact: A rather well-known Konami tune called Morning Music was made to provide a backdrop to this warm-up process for their Bubble System (yes, Konami tried their hand at using Bubble Memory during the mid 80's).
* Reads were destructive. You actually had to write the data back in as you read it out or things got ugly (which meant if the power got cut off between the two processes...you were screwed.
* It seemed quick at the time until advances in DRAM leapfrogged bubble memory.
Now. Flash memory has its drawbacks but they're not nearly as severe as it was with bubble memory. The problem of cell life and longevity has been addressed pretty well with wear management technology, and its current use case precludes DRAM overtaking it; it'll require a different technological leap to defeat it; one likely to sweep up both flash and DRAM in one fell swoop. As noted, its big drawback is price parity, and its speed advantage currently only makes its cost premium worthwhile in limited areas. Here, we're still in WIP territory.