Why the result is still null after placing the condition also?

Question

My original query is given below. even after placing condition 'is null', am supposed to get '2100-12-31' as output. but it's resulting NULL Can any body please help me.

case ty.[ASSC_3PRTY_END_DT]
          when  NULL THEN  '2100-12-31'
          ELSE replace(convert(varchar,ty.[ASSC_3PRTY_END_DT], 111),'/','-')
      END AS thirdPartyAccessLastDate,

output:

 NULL

full query is :

DECLARE @STUDENTID VARCHAR(10)='804403895'

SELECT ty.[STU_ID],
       ty.UCLA_3PRTY_ID,
       ty.ASSC_3PRTY_DESC,
       dr.FIRST_NAME_3PRTY,
       dr.LAST_NAME_3PRTY,
       dr.EMAIL_ADDR_3PRTY,
      replace(convert(varchar,ty.[ASSC_3PRTY_STRT_DT], 111),'/','-') thirdPartyAccessStartDate,
      case ty.[ASSC_3PRTY_END_DT]
          when  NULL THEN  '2100-12-31'
          ELSE replace(convert(varchar,ty.[ASSC_3PRTY_END_DT], 111),'/','-')
      END AS thirdPartyAccessLastDate,
      case when ty.[ASSC_3PRTY_END_DT] IS NULL or ty.[ASSC_3PRTY_END_DT] > GETDATE() then  'Y'
           else 'N'
      end as thirdPartyAccessActive
FROM DBO.SI0_STU_3PRTY ty  left join dbo.SI0_3PRTY_DIR dr on ty.UCLA_3PRTY_ID=dr.UCLA_3PRTY_ID
where ty.STU_ID = @STUDENTID