should i expected performance degradation over time

Sometimes I read that postgres performance is degraded over the time and
something people talk about backup and restore database solve the problem.

It is really true?

I have postgres 9.0 on a windows machine with The autovacuum is ON

I have some configuration tables

And a couple of transactional table.

Transactional table has about 4 millions of rows inserted per day.

In the midnight all rows are moved to a historical table and in the
historical table rows are about 2 months, any transaction older than 2
months are deleted daily.

So, my question is, if Should I expect same performance over time (example:
after 1 year) or should I expect a degradation and must implements come
technics like backup restore every certain time?

Thanks!!