Fixing Brexit-day is a bad idea, for several reasons:
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Most importantly, it removes one of the few remaining areas of flexibility in #A50 process. Regardless of preferred outcome, having option to move date might be useful
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Most obviously, you might find events suddenly require more time. Maybe you're on edge of an agreement, but time runs out. Or perhaps a ratifying body throws a sml spanner in works
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Remember that 29 March 2019 is already wired into process as end of #A50, so UK legislation only makes it even harder to get an extension. Has never been solely for UK to decide to extend
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From EU27 side, hard-wiring will also reduce UK flexibility, which either means EU can push harder on 'take-it-or-leave-it' strategy or increases doubts about viability of any deal. Neither option good for UK
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Overall, this compromises UK position, with little obvious benefit to May domestically, unless it's a balance/cover to some #A50 substantive concession
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