Yes he was utterly destroyed by both magma and rocket launcher fire.
But while this Wesker is dead, those that remain, as part of the Wesker project, may or may not appear as new antagonists (they won't be named Albert though). Another note/possibility to make is that Wesker always has a backup plan, and if he can return once from the dead, who's to say he won't do it again
Wesker got stabbed through the stomach, he survived. Wesker got blown up in the face, he survived. Wesker was burning in lava, yet he survived. Then Capcom decides to make the Rockets pierce his face yet blow up in the lava, not on him. What does this mean? He is likely to have survived yet again.
As well, Capcom has already reported that if people really demand him back, they could eventually put him into another resident evil.