Will 1090 Mhz band become too congested in the future?
There are some areas where the 1090 MHz frequency is already very busy. The area around Frankfurt am Main in Germany and the Los Angeles Basin airspace is the USA are the most congested in the world. 

The direct of impact of ADS-B is an increase in the frequency use of 1090 MHz. At the same time it allows a reduction of number of Mode-S radars, it enables the development of a more effective TCAS with reduced interrogation rates (known as ACAS X), and it reduces the number of interrogations needed by WAM systems. On the long term the impact of ADS-B will likely be positive. 
But with the increase in number of flights the 1090 MHz congestion will remain a concern. 

There are many discussions in the ATC surveillance circles about the topic and the 1090 MHz frequency (and 1030 MHz as well) is are frequently monitored. There are ground based monitors and periodically there are measurement flights to identify areas of high frequency load. Adding new radars to already congested areas is made very difficult by spectrum protection regulations, instead cross-border surveillance data sharing is encouraged. Some countries monitor 1090 MHz transmissions around the clock and the are plans to have a permanent monitoring service of 1030 MHz and 1090 MHz across Europe that will analyse spectrum usages trends and identify rogue transmission.