Once in the new stadium, Spurs will have more money to spend on player purchases and wages than they do now. Remember that they have a net spend on transfer fees of nigh on zero over the past 7-8 years. So there will be no change for the worse in that respect.
Agreed that getting to and from the stadium won't be easy with an extra 26K fans. But the idea is that the added attractions of the new stadium and eventual wider development will encourage fans to stay longer after games. I can't see that working for night games or if Spurs have lost but it should help to ease some of the congestion after weekend games. Added to which, there have been track improvements at Northumberland Park - which will mean double the number of trains from that station - and there are ongoing station and train improvements at White Hart Lane, which will mean the capacity to move a greater number of people in any given period. Further ahead (2030 and on), it is expected that Northumberland Park will be one of the stations on Crossrail 2.