To the very best of my knowledge, mechanical engineering is nothing like a dying industry! If you want to become a mechanical engineer, go for it - there are plenty of jobs for skilled engineers.
However, if you have researched it properly and decided you would prefer civil engineering/architecture then fair enough - however civil engineering and architecture are very different. For civil engineering, you won't need geography and maths & physics will really be the main subjects, just like for mechanical engineering, and the course will last 3 or 4 years. Architecture, on the other hand, is a rather different field - not one which I could comment on the entrance requirements for - and requires rather longer studying (off the top of my head, possibly 7 years, but you would have to check). Make sure you are clear on which one you are interested in if you decide one of those fields interests you more than mechanical engineering.