I can't think of a case of a system like that being revived after 10 or more years. My best guess is that the mismatch between the environment the language was

I can't think of a case of a system like that being revived after 10 or more years. My best guess is that the mismatch between the environment the language was written in and the modern world simply becomes too great for anyone to care after a while. For instance, I have to deal with JSON. There's no reason older languages can't deal with JSON, but it'll often be inconvenient as the libraries try to deal with mismatches in the type system or whatever, and it just can't ever be quite as slick as a more modern language written with some partial, vague combination of the fact that JSON exists in mind, together with the sorts of techniques used to implement JSON support in modern languages. Or sometimes it'll just be the dominant library for dealing with it just has an old design that baked bad assumptions in, and since it's the dominant library it's the one everything uses, so I have to deal with that everywhere. And it's just the death of a thousand of those cuts. "But you can do JSON in my favored language!" Yeah, but it's just not quite as slick. "But we have an HTTP client in this language!" Yeah, but it's just not quite as slick. "But we have GPU support in my favorite language!" Yeah, but it's just not quite as slick. And you've ground all that "not quite as slick" deeply into your language over the decades. So nobody has any one reason they can point at as to why Ada isn't going come back, or why E isn't going to come back, or why Cyclone isn't going to come back... but the reality is that the sum total of the issues is more than sufficient to prevent it from ever happening, and what's going to happen is a more modern language is going to get the support. Advocates also try to "defeat in detail", in analogy to the military technique, arguments to the contrary, but the problem is that while that may satisfy the advocate, everyone else is still left looking at a metaphorical battlefield that has an exhausting number of "battles" over basic deficiencies, and being "defeated in detail" by an advocate just transfers to exhaustion with the underlying language anyhow. It probably isn't a "the issue" with Ada/SPARK. It's a horde of small little things, the sum total of which is too much hassle for anyone to want to deal with.