My question is, what if all of a sudden a big crisis occurs like a dollar collapse and millions will buy bitcoin all at once.
Can the current bitcoin infrastructure handle the sudden amount of transactions?
I fear that bitcoin will collapse because people will see that it takes months to do a simple transaction because of the overload.

So to handle the overload we will need to increase block size limit, but will it be healthy/possible to increase block size limit in a few orders of magnitudes, in a short period of time, without ruining the decentralization of bitcoin?

I am afraid that we need our lousy economy to last a few more years until bitcoin is strong enough.